NCBC-575: Ensure finalization only occurs on non-disposed connections

Change-Id: I25f7e718936d671a1227da4ce1e346fa2129dd81
Reviewed-on: http://review.couchbase.org/40342
Tested-by: Jeffry Morris <jeffrymorris@gmail.com>
Reviewed-by: Matt Ingenthron <matt@couchbase.com>