Pools still thinks a miner is connected after I disconnect
### Description

The pool still thinks there are miners connected despite me having disconnected all my miners.

### Steps to Reproduce

1. Connect a miner to the pool
2. Mine
3. Close the miner
4. Wait a bit
4. Check pool stats in API or poolstats table

### Expected behavior

Show 0 miners after the only miner disconnects.

### Actual behavior

Pool shows 1 miner.

### Reproduces how often

Every time. Tested with a rvn and btcp pool.

### Version

Release 24

### Additional Information

The minerstats table correctly stops recording any data after I disconnect my miners:

SELECT * FROM "public"."minerstats" order by ID desc  limit 4;
````
id	poolid	miner	worker	hashrate	sharespersecond	created
6479	BTCP	b1CyV5r84i1YibEmNiezfUhpngsihV28DVy	pc1	357.427258013331	0.0995529193781611	2018-5-20 19:53:27.569878
6478	BTCP	b1CyV5r84i1YibEmNiezfUhpngsihV28DVy	pc2	1541.1613497793	0.108603184776176	2018-5-20 19:53:27.569878
6477	BTCP	b1CyV5r84i1YibEmNiezfUhpngsihV28DVy	pc1	333.452490731996	0.0928753142969619	2018-5-20 19:48:27.545978
6476	BTCP	b1CyV5r84i1YibEmNiezfUhpngsihV28DVy	pc2	1317.71725576657	0.0928753142969619	2018-5-20 19:48:27.545978
````

But notice how the pool still thinks there's an active miner almost 2 hours after the last timestamp in minerstats. It just loops the last hashrate over and over.

SELECT * FROM "public"."poolstats" where poolid = 'BTCP' order by ID desc limit 30;
````
id	poolid	connectedminers	poolhashrate	sharespersecond	networkhashrate	networkdifficulty	lastnetworkblocktime	blockheight	connectedpeers	created
12724	BTCP	1	1899	0	5261174	26681346.9818729	2018-5-20 21:38:01.609811	324095	8	2018-5-20 21:38:28.023264
12722	BTCP	1	1899	0	5261174	26186252.13143	2018-5-20 21:30:24.05706	324093	8	2018-5-20 21:33:28.017282
12720	BTCP	1	1899	0	5336150	26093578.1944954	2018-5-20 21:26:47.509213	324092	8	2018-5-20 21:28:27.997063
12718	BTCP	1	1899	0	5336150	26230294.0058269	2018-5-20 21:22:00.613767	324091	8	2018-5-20 21:23:27.952288
12716	BTCP	1	1899	0	5444354	26433824.5463173	2018-5-20 21:15:01.624419	324090	8	2018-5-20 21:18:27.932414
12714	BTCP	1	1899	0	5444354	25998176.4293023	2018-5-20 21:11:21.609836	324089	8	2018-5-20 21:13:27.912642
12712	BTCP	1	1899	0	5380522	25885174.8090356	2018-5-20 21:08:07.810208	324086	8	2018-5-20 21:08:27.892538
12710	BTCP	1	1899	0	5380522	25525220.3172569	2018-5-20 21:02:37.154592	324084	8	2018-5-20 21:03:27.866323
12708	BTCP	1	1899	0	5429869	26917394.286106	2018-5-20 20:49:41.919934	324078	8	2018-5-20 20:58:27.860783
12706	BTCP	1	1899	0	5429869	26917394.286106	2018-5-20 20:49:41.919934	324078	8	2018-5-20 20:53:27.855288
12704	BTCP	1	1899	0	5492798	26415725.0049041	2018-5-20 20:41:12.053473	324076	8	2018-5-20 20:48:27.82789
12702	BTCP	1	1899	0	5492798	26415725.0049041	2018-5-20 20:41:12.053473	324076	8	2018-5-20 20:43:27.807447
12700	BTCP	1	1899	0	5415411	26838467.5208182	2018-5-20 20:34:26.880795	324074	8	2018-5-20 20:38:27.778781
12698	BTCP	1	1899	0	5415411	26932806.5494406	2018-5-20 20:32:49.138021	324073	8	2018-5-20 20:33:27.758805
12696	BTCP	1	1899	0	5394974	27687835.3044748	2018-5-20 20:28:12.35289	324071	8	2018-5-20 20:28:27.735014
12694	BTCP	1	1899	0	5394974	27424628.3472378	2018-5-20 20:22:55.422745	324068	8	2018-5-20 20:23:27.703029
12692	BTCP	1	1899	0	5172963	27518921.4743178	2018-5-20 20:17:09.910131	324067	8	2018-5-20 20:18:27.68311
12690	BTCP	1	1899	0	5172963	27690014.330306	2018-5-20 20:09:38.045493	324063	8	2018-5-20 20:13:27.663027
12688	BTCP	1	1899	0	5234339	27262618.5046878	2018-5-20 20:06:25.385782	324062	8	2018-5-20 20:08:27.635975
12686	BTCP	1	1899	0	5234339	27180733.3473665	2018-5-20 20:02:55.209426	324061	8	2018-5-20 20:03:27.616343
12684	BTCP	1	1899	0	5314810	28176801.5446721	2018-5-20 19:57:19.47917	324059	8	2018-5-20 19:58:27.594402
12682	BTCP	1	1899	0	5314810	27093912.9351059	2018-5-20 19:53:21.288886	324057	8	2018-5-20 19:53:27.569878
12680	BTCP	1	1652	0	5243065	26113099.2269702	2018-5-20 19:41:40.289039	324055	8	2018-5-20 19:48:27.545978
12678	BTCP	1	1700	0	5243065	26113099.2269702	2018-5-20 19:41:40.289039	324055	8	2018-5-20 19:43:27.521456
12676	BTCP	1	1676	0	5129995	25525812.9003471	2018-5-20 19:37:49.001722	324054	8	2018-5-20 19:38:27.497268
12674	BTCP	1	1651	0	5129995	23824027.5132864	2018-5-20 19:30:30.278372	324046	8	2018-5-20 19:33:27.465387
12672	BTCP	1	1697	0	4973748	23083828.952127	2018-5-20 19:28:09.927815	324044	8	2018-5-20 19:28:27.44073
12670	BTCP	1	1624	0	4973748	21380513.7545253	2018-5-20 19:22:43.011837	324040	8	2018-5-20 19:23:27.416582
12668	BTCP	1	1541	0	4954255	20834343.1950582	2018-5-20 19:17:52.531227	324039	8	2018-5-20 19:18:27.392649
12666	BTCP	1	1461	0	4954255	19246499.6777142	2018-5-20 19:11:16.91796	324032	8	2018-5-20 19:13:27.361025
````