HR stats cannot be reset fully right after sync's completed
![image](https://user-images.githubusercontent.com/93492811/168493841-55c34aec-56e8-4a5e-aada-96d248b7b618.png)
![image](https://user-images.githubusercontent.com/93492811/168498842-5a83ea23-c207-4d45-983d-e343f6b60711.png)

Here are 2 examples of the latest branch. The 2nd one has a new [msg 'P2POOL'S BEEN SYNCED'](https://github.com/SChernykh/p2pool/issues/147#issuecomment-1127075222) and was got by dropping client connections by FW after some shares had been found and stats had been updated (I'll explain why later).
I tried to look into the problem and found that if I add `m_cumulativeHashes = 0;` to the 'void StratumServer::reset_share_counters()' section, the HR stats will indeed be reset fully:
![image](https://user-images.githubusercontent.com/93492811/168499675-d2c680e4-2bac-4e2d-bf37-e9cc52413b94.png)
![image](https://user-images.githubusercontent.com/93492811/168499772-0a8ff7ea-5b6b-4204-98f3-501d277b556d.png)

BUT there may sometimes be another situation that interferes with a normal reset! Look at this screen:
![image](https://user-images.githubusercontent.com/93492811/168500149-7e1d042e-24ca-45ba-b230-d2005a5cd8c2.png)
We can see some "intra-" shares, the new "p2pool sync" message (that must reset all share stats) and... a share that had been found right before the sync message, which was added to the stats later when the real diff had already been too high because of the main chain sync! And another share was also found right after the sync and it even didn't have a client name and ip:port... I think there's a need in extra "low diff check" or something like this...
Total hashes can NOT be reset as I suggested above (compare the hashes sum before and after the sync). And I can't even suppose what to do with the encreased total share counter value!

p.s. I've just got another example (I haven't interfered with my serv now by writin 'status' and so on):
![image](https://user-images.githubusercontent.com/93492811/168502427-804af8c3-a202-4090-b69d-b7318295a8db.png)
Triple number of shares received in one second with the sync message!