Fix bug that processAccount operates on same epoch storage data for every epoch

[0x-r4bbit]

Something we've discovered recently.
There's a bug in the for loop we use in processAccount.
Namely this line:

https://github.com/logos-co/staking/blob/develop/contracts/StakeManager.sol#L444

Because we're doing the storage iEpoch = epochs[userEpoch] assignment inside of for (...) that assignment happens only once. Meaning for every iteration, we're updating the same epoch object.

Instead we need to update the storage pointer with every iteration.
The fix for this issue should include a test that proofs it's working as intended.