Add Couchbase.Core.Exceptions.TimeoutException

There is a name collision between System.TimeoutException and Couchbase.Core.Exceptions.TimeoutException, so previously the type was omitted and UnambiguousTimeoutException and AmbiguousTimeoutException both derived from CouchbaseException. This ticket to add Couchbase.Core.Exceptions.TimeoutException and simply deal with the collision by fully qualifying the namespace. This is for consisteny with other sdks and alignment with the RFC.