Removed commit process synchronization

which was added for reducing problems occuring when multiple threads
updated the store simultaneously. Nowadays there's a synchronization on
the store application itself, which guards for even more scenarios than
these ever did, since the store applier is only one instance for a
datbaase instance, whereas commit process instances are aplenty.

Removing these synchronizations also re-enables the ability to implement
batched writes to the transaction log.
Merge pull request #2891 from tinwelint/2.2-remove-commit-proc-sync

Removed commit process synchronization