go/zodb: Connection.DB

Provide way for users to retrieve Connection -> DB from which it was opened.
Wendelin.core 2 might need this to open a second connection given a one.

go/zodb/connection.go

-// Copyright (C) 2018-2019  Nexedi SA and Contributors.
+// Copyright (C) 2018-2020  Nexedi SA and Contributors.
 //                          Kirill Smelkov <kirr@nexedi.com>
 //
 // This program is free software: you can Use, Study, Modify and Redistribute
@@ -190,6 +190,12 @@ func newConnection(db *DB, at Tid) *Connection {
 	}
 }
 
+// DB returns database handle under which the connection was opened.
+func (conn *Connection) DB() *DB {
+	conn.checkLive("db")
+	return conn.db
+}
+
 // At returns database state corresponding to the connection.
 func (conn *Connection) At() Tid {
 	conn.checkLive("at")