*index: support --commit=$SECONDS to adjust the commit interval

*index: support --commit=$SECONDS to adjust the commit interval

The default 5s interval is too low for initial indexing with
gian inboxes or extindices which aren't ready for public
consumption, yet.  It's also too low for --wal users since
WAL mode of SQLite (by default) allows readers to proceed
without waiting on a writer.

I'm not sure what the defaults should be, so allow users to
change it, for now.