fix bug that prevented db being written to disk in SingleWriter mode

fix bug that prevented db being written to disk in SingleWriter mode

The bug occurred when closeDb was not called, and garbage collection of
the DbHandle didn't give the workerThread time to shut down. Fixed by
exiting the runSqlite action when a commit is made.

(MultiWriter mode already forked off a runSqlite action, so avoided the
problem.)

This commit was sponsored by Brock Spratlen on Patreon.