Do not provide jobs to stratum clients until synchronized to highest known side-chain height
In the current implementation, p2pool will provide jobs immediately on startup to stratum clients, even if it is still syncing with the side-chain. This causes clients to get job spam and false shares to be "found" and logged with a diff of `100000` or `111111`.

It would be ideal if p2pool paused job sending to stratum clients until fully synced with highest known side-chain height.

Example false shares found while p2pool was starting/syncing/verifying blocks:

```
NOTICE  2021-09-08 12:37:31.6415 StratumServer SHARE FOUND: mainchain height 2444927, diff 100000, effort 10.001%
NOTICE  2021-09-08 12:38:43.1568 StratumServer SHARE FOUND: mainchain height 2444928, diff 100000, effort 60.006%
NOTICE  2021-09-08 12:38:43.8692 StratumServer SHARE FOUND: mainchain height 2444928, diff 100000, effort 10.001%
NOTICE  2021-09-08 12:39:27.8570 StratumServer SHARE FOUND: mainchain height 2444929, diff 100000, effort 90.009%
NOTICE  2021-09-08 12:40:35.9209 StratumServer SHARE FOUND: mainchain height 2444930, diff 100000, effort 90.009%
NOTICE  2021-09-08 12:40:42.9515 StratumServer SHARE FOUND: mainchain height 2444930, diff 100000, effort 20.002%
NOTICE  2021-09-08 13:08:58.2760 StratumServer SHARE FOUND: mainchain height 2444942, diff 100000, effort 110.002%
NOTICE  2021-09-08 13:09:14.0703 StratumServer SHARE FOUND: mainchain height 2444942, diff 100000, effort 110.002%
NOTICE  2021-09-08 13:09:27.6229 StratumServer SHARE FOUND: mainchain height 2444942, diff 100000, effort 100.001%
NOTICE  2021-09-08 16:25:32.0452 StratumServer SHARE FOUND: mainchain height 2445024, diff 100000, effort 100.001%
NOTICE  2021-09-08 16:25:47.8264 StratumServer SHARE FOUND: mainchain height 2445024, diff 100000, effort 10.001%
NOTICE  2021-09-08 16:26:02.2404 StratumServer SHARE FOUND: mainchain height 2445024, diff 100000, effort 110.002%
NOTICE  2021-09-08 17:37:23.8036 StratumServer SHARE FOUND: mainchain height 2445071, diff 100000, effort 100.001%
NOTICE  2021-09-08 17:37:43.1492 StratumServer SHARE FOUND: mainchain height 2445071, diff 100000, effort 130.004%
NOTICE  2021-09-08 17:37:58.6347 StratumServer SHARE FOUND: mainchain height 2445071, diff 100000, effort 30.003%
NOTICE  2021-09-09 13:13:05.2804 StratumServer SHARE FOUND: mainchain height 2445633, diff 100000, effort 100.001%
NOTICE  2021-09-09 13:13:19.0744 StratumServer SHARE FOUND: mainchain height 2445633, diff 100000, effort 110.002%
NOTICE  2021-09-09 13:13:24.8260 StratumServer SHARE FOUND: mainchain height 2445633, diff 111111, effort 117.004%
NOTICE  2021-09-09 13:13:36.1705 StratumServer SHARE FOUND: mainchain height 2445633, diff 100000, effort 110.002%
NOTICE  2021-09-09 13:13:52.7460 StratumServer SHARE FOUND: mainchain height 2445633, diff 100000, effort 110.002%
NOTICE  2021-09-09 13:13:52.7495 StratumServer SHARE FOUND: mainchain height 2445633, diff 100000, effort 0.000%
NOTICE  2021-09-09 13:13:52.7528 StratumServer SHARE FOUND: mainchain height 2445633, diff 100000, effort 0.000%
```