MB-30916: Don't free closed-checkpoint-memory under lock (2)
MB-30916: Don't free closed-checkpoint-memory under lock (2)

In http://review.couchbase.org/#/c/98468/ I wanted to fix the
performance regression introduced in
http://review.couchbase.org/#/c/98301/ (checkpoint memory released under
CM::queueLock), but after that fix memory is still freed under lock.

At last, here I free checkpoint memory after the lock has been released.

This regression has been known for a while, and recenlty also spotted by
the Perf team in MB-31834.

Change-Id: I59cc9107defcec10ab876e02f5a4cb4d89c8c656
Reviewed-on: http://review.couchbase.org/98714
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Dave Rigby <a09264da4832c7ff1d3bf1608a19f4b870f93750@couchbase.com>

daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine