MB-22016: Defragmenter test tweaks
MB-22016: Defragmenter test tweaks

* try and reduce the heap churn during the test by using the stack
  and DocKey.
* don't run the DefragmenterTest.MappedMemory test under valgrind
  as it appears to trigger valgrind issues in jemalloc.

Note that DefragmenterTest.MappedMemory will fail some EXPECT
tests because of the following config options which are changed
by running on valgrind.

JEMalloc - no valgrind
  opt.quarantine: 0
  opt.redzone: false

JEMalloc - valgrind
  opt.quarantine: 1677721
  opt.redzone: true

Change-Id: Ia7122121183ddeddf5b6edaadeddc5bab95daae6
Reviewed-on: http://review.couchbase.org/71096
Reviewed-by: Trond Norbye <60edd2ef23891a753f231b0c6f161dc634079a93@gmail.com>
Tested-by: buildbot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>

couchbase/ep-engine,daverigby/ep-engine,membase/ep-engine,daverigby/kv_engine,membase/ep-engine,daverigby/ep-engine,daverigby/ep-engine,membase/ep-engine,couchbase/ep-engine,daverigby/kv_engine,daverigby/kv_engine,couchbase/ep-engine,daverigby/kv_engine,membase/ep-engine,daverigby/ep-engine,couchbase/ep-engine