DCB callbacks for the write queue water marks should be usable by several routers of a session

A DCB has a single callback for each of the two write queue water marks. The client DCB is shared by all routers of a session, for example the children of a SmartRouter session, and it already has the core's default callbacks for throttling the backend connections.

A router that registers its own callback replaces the default one, and thereby changes the behaviour of the whole session. Two routers of the same session that both register callbacks overwrite each other. A router that wants to react to a slow client therefore has to avoid the callbacks, as the ODBC routers do by polling the write queue.

The callback arrangement should let routers observe the water marks without interfering with each other or with the core's throttling.