Utilize disk_log for search backend sync

[scohen]

Whenever we sync, we have to write the entire database to the disk, this can be hundreds of megs and can take some time. Instead, we should write deltas using disk_log, and load them after we load the table. We can then rewrite the entire index much more occasionally in a background job.

We can also remove the explicit calls to sync in the search store and let the backend handle writing its state.