Instruct quartz to use all daemon threads.

Bugs closed: AURORA-560

Reviewed at https://reviews.apache.org/r/23219/
Export a stat to track number of LOST gc_executors

Bugs closed: AURORA-562

Reviewed at https://reviews.apache.org/r/23180/
Configure H2 to use a named (non-private) database, and not close the database.

Bugs closed: AURORA-573

Reviewed at https://reviews.apache.org/r/23273/
Documentation for task health checking and graceful shutdown.