Merged the current state of the garbage collection branch which improves the...

Merged the current state of the garbage collection branch which improves the database cleanup stage of the maintenance
and improves the reactiveness of the maintenance overall. We ported the way items are processed to a queue based method
that can use the CPUs more effectively and does not create thousands of threads.

BUGS: 283062 216895 374225
CCBUGS: 351658 362023 329353
FIXED-IN: 5.5.0