shell's last_epoch field should only be updated during block commit

[tzemanovic]

There are 2 fields for epoch in storage - last_epoch and block.epoch , which were both being updated in ABCI EndBlock, but the last_epoch must be atomic - it can only be changed when the block is being committed (ABCI Commit). This was caught in the e2e tests in anoma/anoma#1234, because a MASP tx must be applied in the same epoch as it’s constructed, but because the last_epoch was being updated before commit, an RPC epoch query that could make it between the EndBlock and Commit would return the new epoch too early (this is timing sensitive, so not very deterministic)