Updates isPrunable to be getPruneAction which lets caller know if
message should be pruned (original function) or would cause a prune
action (added functionality). When applicable we prune for the given
FID after the message is merged.

Prune job is still running in the background but can be removed
in the future once it's confirmed there's no stragglers.

Benchmarking with production data, this change seems to add a 5%
overhead on average. The FID lock added to Store::prune_messages
is required for performance reasons. Without the lock, the overhead
is run dependent and takes 100%-500% longer.