(2.2) billing: improve the insert queue logic

version for 2.2 of http://rb.dcache.org/r/5992

See that patch for details.

Target: 2.2
Request: 2.2
Patch: http://rb.dcache.org/r/5993
Require-notes: yes
Require-book: yes

RELEASE NOTES:
BOOK:

An improved insert strategy for the billing database has been implemented.  The following changes to billing properties should be noted:

billingMaxInsertsBeforeCommit
billingMaxTimeBeforeCommitInSecs

(one-of?org.dcache.services.billing.db.impl.DirectQueueDelegate)billing.db.inserts.queue-delegate.type=org.dcache.services.billing.db.impl.DirectQueueDelegate

billing.db.inserts.max-queue-size=100000

billing.db.inserts.max-batch-size=1000

(one-of?true|false)billing.db.inserts.drop-messages-at-limit=true

It should be noted that, in conjunction with http://rb.dcache.org/r/5983, which modifies the database triggers, insertion should now perform without message loss (and without running out of memory) under most conditions (3000Hz or less).