Rebalancing is unsafe to use with readwritesplit

Readwritesplit and readconnroute store a reference to a thread-local value and use it for the duration of the whole session. If a rebalancing occurs, the reference will point to a thread-local value on another thread. This can result in crashes or memory leaks if rebalancing is used with readwritesplit. With readconnroute this will only result in some statistics being wrong.