org-roam-id-get-file no longer falls back onto the current buffer: It queries the org-roam database, and optionally org-id-locations, and declares that it does not exist if it isn't in either.
The high value has been reported to cause significant slowdowns, so we
reset the default, and add documentation for its customization instead.
Ref #834
We use sqlite transactions to commit changes into the database, rather
than storing all the data in a list before running one big insert.
Hopefully this gives a noticeable perf boost.
We also add `org-roam-db-gc-threshold`, which shaves time by deferring the garbage collection to the end.
