[BTCP] Insufficient shielded balance for payment
### Description

Running a BTCP pool for my miners. Found a block, and I get a payment error in the logs despite the payment really being done on the blockchain. There is also no entries logged in the payments table but the balance_changes table contains a "Balance reset after payment" entry.

### Steps to Reproduce

1. Setup conf.json for BTCP with a z and non-z address as required.

### Expected behavior

Payment going through OK and being logged in the DB.

### Actual behavior

```
[2018-05-16 05:41:00.8658] [I] [PayoutManager] Processing payments for pool BTCP
[2018-05-16 05:41:00.8740] [I] [BTCP] [ZCash Payout Handler] Unlocked block 321435 worth 1.56261 BTCP
[2018-05-16 05:41:00.8740] [I] [PayoutManager] Processing payments for pool BTCP, block 321435
[2018-05-16 05:41:00.8767] [I] [BTCP] Adding 0.01547 BTCP to balance of b1CyV5r84i1YibEmNiezfUhpngsihV28DVy
[2018-05-16 05:41:00.9008] [I] [PPLNS Payment] Fetching page 0 of shares for pool BTCP, block 321435
[2018-05-16 05:41:01.2377] [I] [PPLNS Payment] Fetching page 1 of shares for pool BTCP, block 321435
[2018-05-16 05:41:01.5508] [I] [PPLNS Payment] Balance-calculation for pool BTCP, block 321435 completed with accumulated score 0.4067 (20.3%)
[2018-05-16 05:41:01.5523] [I] [PPLNS Payment] Adding 0.31459 BTCP to balance of b1CyV5r84i1YibEmNiezfUhpngsihV28DVy for 12.3 M (12295760) shares for block 321435
[2018-05-16 05:41:01.5642] [I] [PPLNS Payment] 12.3 M (12295760) shares contributed to a total payout of 0.31459 BTCP (20.33% of block reward) to 1 addresses
[2018-05-16 05:41:01.5914] [I] [BTCP] [ZCash Payout Handler] Shielding ZCash Coinbase funds
[2018-05-16 05:41:01.6039] [I] [BTCP] [ZCash Payout Handler] z_shieldcoinbase operation id: opid-e70c9340-a100-4c50-86c7-e8e8fda15c36
[2018-05-16 05:41:01.6087] [I] [BTCP] [ZCash Payout Handler] Waiting for shielding operation completion: opid-e70c9340-a100-4c50-86c7-e8e8fda15c36
[2018-05-16 05:41:02.8824] [I] [BTCP] [Pool] [0HLDORU6H6EIE] Share accepted: D=96
[2018-05-16 05:41:06.6087] [I] [BTCP] [Pool] Broadcasting job
[2018-05-16 05:41:07.1223] [I] [BTCP] [Pool] [0HLDORU6H6EIF] Share accepted: D=397
[2018-05-16 05:41:07.1223] [I] [BTCP] [Pool] [0HLDORU6H6EIF] VarDiff update to 398
[2018-05-16 05:41:11.6259] [I] [BTCP] [ZCash Payout Handler] Waiting for shielding operation completion: opid-e70c9340-a100-4c50-86c7-e8e8fda15c36
[2018-05-16 05:41:12.4382] [I] [BTCP] [Pool] [0HLDORU6H6EIE] Share accepted: D=96
[2018-05-16 05:41:14.9796] [I] [BTCP] [Pool] [0HLDORU6H6EIE] Share accepted: D=96
[2018-05-16 05:41:17.4005] [I] [BTCP] [Pool] Broadcasting job
[2018-05-16 05:41:21.6383] [I] [BTCP] [ZCash Payout Handler] Waiting for shielding operation completion: opid-e70c9340-a100-4c50-86c7-e8e8fda15c36
[2018-05-16 05:41:25.6914] [I] [BTCP] [Pool] [0HLDORU6H6EIF] Share accepted: D=398
[2018-05-16 05:41:26.6598] [I] [BTCP] [Pool] Broadcasting job
[2018-05-16 05:41:27.3614] [I] [StatsRecorder] Updating hashrates for pool BTCP
[2018-05-16 05:41:27.5282] [I] [BTCP] [Pool] [0HLDORU6H6EIF] Share accepted: D=398
[2018-05-16 05:41:28.1418] [I] [BTCP] [Pool] [0HLDORU6H6EIE] Share accepted: D=96
[2018-05-16 05:41:31.6401] [I] [BTCP] [ZCash Payout Handler] Waiting for shielding operation completion: opid-e70c9340-a100-4c50-86c7-e8e8fda15c36
[2018-05-16 05:41:34.7247] [I] [BTCP] [Pool] [0HLDORU6H6EIE] Share accepted: D=96
[2018-05-16 05:41:35.9271] [I] [BTCP] [Pool] [0HLDORU6H6EIF] Share accepted: D=398
[2018-05-16 05:41:37.0181] [I] [BTCP] [Pool] Broadcasting job
[2018-05-16 05:41:41.6424] [I] [BTCP] [ZCash Payout Handler] Waiting for shielding operation completion: opid-e70c9340-a100-4c50-86c7-e8e8fda15c36
[2018-05-16 05:41:46.6021] [I] [BTCP] [Pool] [0HLDORU6H6EIE] Share accepted: D=96
[2018-05-16 05:41:46.6550] [I] [BTCP] [Pool] Broadcasting job
[2018-05-16 05:41:50.8477] [I] [BTCP] [Pool] [0HLDORU6H6EIE] Share accepted: D=96
[2018-05-16 05:41:51.4839] [I] [BTCP] [Pool] [0HLDORU6H6EIE] Share accepted: D=96
[2018-05-16 05:41:51.6441] [I] [BTCP] [ZCash Payout Handler] Waiting for shielding operation completion: opid-e70c9340-a100-4c50-86c7-e8e8fda15c36
[2018-05-16 05:41:56.7066] [I] [BTCP] [Pool] Broadcasting job
[2018-05-16 05:42:01.6533] [I] [BTCP] [ZCash Payout Handler] z_shieldcoinbase successful
[2018-05-16 05:42:01.6561] [I] [BTCP] [ZCash Payout Handler] Insufficient shielded balance for payment of 0.33006 BTCP
```

### Reproduces how often

Just found 1 block so far.

### Version

Release 24

### Additional Information

My wallet says I have the remaining funds, so not sure why it's complaining about insufficient shielded balance.

```
[btcp@btcp ~]$ btcp-cli z_listaddresses
[
  "zkJJzUnpXDkyfgR2aGGMU1Fjb6W6UmQ4SHGArT3Bwnd9xewcqv62sAN7zW5xsjKdvcHKZ75UqBP2iphkV2otAUPvt2iFADG"
]

[btcp@btcp ~]$ btcp-cli z_getbalance "zkJJzUnpXDkyfgR2aGGMU1Fjb6W6UmQ4SHGArT3Bwnd9xewcqv62sAN7zW5xsjKdvcHKZ75UqBP2iphkV2otAUPvt2iFADG"
1.23235460
```

I also don't have any accounts with names in my non-z addresses:

```
[btcp@btcp ~]$ btcp-cli listaccounts
{
  "": 0.00000000
}
```

But I noticed the non-z address changed by itself. (I'm thinking maybe like bitcoin it gives a new address after every transaction) But not sure if relevent to this problem.

```
[btcp@btcp ~]$ btcp-cli getaccountaddress ""
b1EYBP7jNi3UyY2jAm1RYEibdUJrHUuWo6c
```
^^ not the non-z address in my json.conf
