LogFactory: Silence expected warning in test
RaftConsensus: Set storageDirectory regardless of storageModule

It's needed for snapshots anyhow, and most storage modules will need it.
Benchmark: fix stray word in usage
Rework Log::Sync interface to be more efficient and flexible

Now the Log accumulates a Sync object until you take it, which allows
for better batching to disk on leaders. There's also a call back into
the Log that is done after waiting with the Raft mutex held, which
allows the log to discard any pinned resources.