CBQE-1148: add monitor db fragmentation task

Change-Id: Ida3dad0fba3e8aebe65b7b5413d56172a744c107
Reviewed-on: http://review.couchbase.org/25467
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Deepkaran Salooja <deepkaran.salooja@globallogic.com>