billing: add commands for recording queue insert data

Adds commands to billing cell to be able to monitor insert queue statistics and to dump threads.

NOTE:  I have decided not to request back-porting to 2.7 or 2.6 for this patch.

TESTING:

[cmssrv153.fnal.gov] (billing) admin > log commit statistics on
on
[cmssrv153.fnal.gov] (billing) admin > log commit statistics off
off

billing log contains:
16 Sep 2013 11:51:32 (billing) [alm-admin-118] insert queue (last 0, current 0, change 0/minute)
16 Sep 2013 11:51:32 (billing) [alm-admin-118] commits (last 0, current 0, change 0/minute)
16 Sep 2013 11:51:32 (billing) [alm-admin-118] dropped (last 0, current 0, change 0/minute)
16 Sep 2013 11:51:32 (billing) [alm-admin-118] total memory 505282560; free memory 482253512

this repeated every minute until turned off.

Target: master
Patch: http://rb.dcache.org/r/5994/
Require-notes: yes
Require-book: yes
Acked-by: Karsten

RELEASE NOTES:
BOOK:

You can now obtain statistics (printed to billing log and pinboard) via the dcache admin command:
display insert statistics <on/off>

Turning this command on logs the following once a minute:

insert queue (last 0, current 0, change 0/minute)
commits (last 0, current 0, change 0/minute)
dropped (last 0, current 0, change 0/minute)
total memory 505282560; free memory 482253512

"insert queue" refers to how many messages actually were put on the queue; "commits" are the number of messages committed to the database; dropped are the number of lost messages. "last" refers to the figures at the last iteration.  For insert queue, this is the actual size of the queue; for commits and dropped, these are cumulative totals.

You can also generate a Java thread dump by issuing the "dump threads" command.