billing: Improve shutdown for db backend

The billing db would always print a warning on shutdown caused by
an expected thread interrupt. This patch elliminates that warning.
The patch also removes unnecessary checks for null values as well
as failures to react correctly to thread interrupt (when the thread
calling close is interrupted, it shouldn't continue blocking on
yet another Thread#join call). Finally, the patch changes serial
shutdown of consumer threads to shutting them down concurrently.

Target: trunk
Request: 2.10
Require-notes: yes
Require-book: no
Acked-by: Albert Rossi <arossi@fnal.gov>
Patch: https://rb.dcache.org/r/7378/
(cherry picked from commit 50421f49871eb489918f4fe64db3b09de6ec499a)
Merge pull request #965 from gbehrmann/fix/2.10/rb7378

billing: Improve shutdown for db backend