index: do not attach inbox to extindex unless updated
We'll count the number of log changes (regardless of index or
unindex) and only attach inboxes to ExtSearchIdx objects when
they get new work. We'll also reduce lock bouncing and only
update external indices after all per-inbox indexing is done.
This also updates existing v2 indexing/unindexing callers
to be more consistent and ensures unindex log entries update
per-inbox last commit information.