Decouple StoredValue and HashTable class

Decouple StoredValue and HashTable class

A StoredValue object doesn't necessarily have to be part of
a HashTable. Hence, decouple the two

Change-Id: I6b88a3e010494989d3c1ad938c329a230c79d91d
Reviewed-on: http://review.couchbase.org/78170
Reviewed-by: Dave Rigby <daver@couchbase.com>
Tested-by: Build Bot <build@couchbase.com>