MB-21475: Account for memory alloc/dealloc in stopTaskGroup

MB-21475: Account for memory alloc/dealloc in stopTaskGroup

While stopping a task group, any memory allocations/deallocations made
should be accounted to the bucket in question.  Hence no
`onSwitchThread(NULL)` call.

(Note: This is very similar to MB-20054).

Change-Id: Ib5f88fe8e4061ac827a28e267717df32131aba23
Reviewed-on: http://review.couchbase.org/69184
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Daniel Owen <owend@couchbase.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>