JCBC-503: Release observers on shutdown.

Motivation
----------
On shutdown of the provider, there are still observers referenced. Making sure they
are not referenced anymore should be part of a clean shutdown procedure.

Modifications
-------------
Clear out the observers array to remove the references.

Result
------
Cleaner shutdown.

Change-Id: I710d1b8be9dea88aba0fe28162442951807610de
Reviewed-on: http://review.couchbase.org/40263
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Reviewed-by: Matt Ingenthron <matt@couchbase.com>