MB-23795: Ephemeral Tombstone purging Task

MB-23795: Ephemeral Tombstone purging Task

Expands on the previous patch to implement a Task which performs
Tombstone purging. This Task is scheduled periodically (see
ep_ephemeral_metadata_purge_interval), and when run it visits all
vBuckets and purges all applicable OSVs.

Task can be reconfigured dynamically via epctl parameters:

- ephemeral_metadata_purge_age: Age in seconds after which deleted
  items are purged.

- ephemeral_metadata_purge_interval: How often should Tombstone
  purging task run to check for items to be purged.

Example output:

    NOTICE (eph) Ephemeral Tombstone Purger starting with purge age:60
    NOTICE (eph) Ephemeral Tombstone Purger completed. Purged 39000 items. Took 104ms. Sleeping for 60 seconds.

Change-Id: I126c74f2e82c0a31a2843240303548a24af2e90f
Reviewed-on: http://review.couchbase.org/76485
Reviewed-by: Jim Walker <jim@couchbase.com>
Tested-by: Build Bot <build@couchbase.com>