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.
