SPY-138: Allow the ExecutorService to be overridden.

Change-Id: I6f9c577f734c33f2d6004031253dd561b6302b4c
Reviewed-on: http://review.couchbase.org/29373
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Reviewed-by: Matt Ingenthron <matt@couchbase.com>