Support statement pipelining of reads and multi-target queries

This is a continuation of MXS-1245.

With MXS-1245 implemented, queries that are routed to the master are executed in a streaming fashion. This can be relatively easily extended to cover queries that target slaves but the major hurdle with that is the target selection code has to be modified to be aware of it.

For multi-target queries the issue is more complex. In theory this is simple: append the command to the history and execute it on all backends. The replier for all the executed session commands should be the same server. The accounting of the session command history must be updated for this to be doable.

The problems come when different target types are mixed while streaming queries. The streaming of session commands and single-target queries can be done if the replying backend is the same as the replier of the session command.