SPY-137: Properly shutdown executorService on shutdown().

Change-Id: I23bf0a30cc96c5721d6ed0d5c16bdfc2113c0fe4
Reviewed-on: http://review.couchbase.org/29372
Reviewed-by: Matt Ingenthron <matt@couchbase.com>
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>