only commit host WAL if there are changes to commit

I don't understand the host well enough to say whether this change is safe, but we could definitely use this functionality. Currently the host creates, writes, syncs, and renames contractmanager.wal_temp and contractmanager.json_temp every 500ms, even if the host has been sitting idle. With this PR, the host performs no writes unless it's actually recording a change. This should greatly reduce the wear on idle host disks.

Fixes #2215 (closed)