MB-12067: Disable defragmenter by default

Disable the Defragmenter task by default, to allow profiling of just
jemalloc compared to tcmalloc, without the defragmenter being in play.

Change-Id: I87d5c15c217f0f53b8a264ccf17377d12f7c7e82
Reviewed-on: http://review.couchbase.org/43242
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>