NCBC-702: Make Release and AddRef un-callable in assemblies other than Couchbase

and Couchbase.Tests. IRefcountable is an internal interface whose method
implmentations should not be called externally. This commit makes them
(mostly) un-callable from assemblies that are not 'friend' assemblies of
Couchbase.NetClient.

Change-Id: Ie796909a747eaaf002fcf0984624d9ed0589c5d7
Reviewed-on: http://review.couchbase.org/42452
Tested-by: Jeffry Morris <jeffrymorris@gmail.com>
Reviewed-by: Simon Baslé <simon@couchbase.com>