Synchronizes store application

This brings in one more case under synchronization, and that is where a
store applier runs at the same time as a response unpacker. Also less
commit process objects are produced, which means that the synchronization
that guards their commit method does a better job.

bb822c974cdadb333a3f0daeb4ca4d5eca609120 started this work and this commit
takes it a little further. The root cause of why lack of this
synchronization is causing problems has to be found.
Merge pull request #2838 from tinwelint/2.2-sync-store-applier

Synchronizes store application