watchtower/wtmock/client_db: adjust mock clientdb behavior

In advance of the upcoming wtdb.ClientDB, we'll modify the behavior
of the mockdb to be more like the final bbolt backed one, and assert
that all or our tests are still passing.

watchtower/wtdb/client_session.go

@@ -39,6 +39,12 @@ var (
 	// created because session key index differs from the reserved key
 	// index.
 	ErrIncorrectKeyIndex = errors.New("incorrect key index")
+
+	// ErrClientSessionAlreadyExists signals an attempt to reinsert
+	// a client session that has already been created.
+	ErrClientSessionAlreadyExists = errors.New(
+		"client session already exists",
+	)
 )
 
 // ClientSession encapsulates a SessionInfo returned from a successful