Defragmenter: Use ExTask RCPTR

Use a ExTask (reference-counted pointer) for the defragmenterTask to
ensure that there isn't any races between the ExecutorPool using it
and shutting the task down.

Fixes some intermittent unit-test failures seen on OS X.

Change-Id: Icc7bbea8bbdd3fb26c8183106a2568dcb95275d5
Reviewed-on: http://review.couchbase.org/43243
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>
test_memory_limit: Increase document size for TEMP_OOM test

When running under jemalloc we appear to have a lower mem_used for
this test, with the result that occasionally it will fail as the
second store() passes when it's expected to hit TEMP_OOM.

Fix this by increasing the size of the docuement we set.

Change-Id: I8b65f6e138b7ba1544957927d16ffd4d39867aea
Reviewed-on: http://review.couchbase.org/43246
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>