JCBC-366: Properly override ExecutorService and Metrics.

Change-Id: Ia1987d3dc9ca8cf8c73435ba505ada1fd83f265e
Reviewed-on: http://review.couchbase.org/29374
Reviewed-by: Matt Ingenthron <matt@couchbase.com>
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>