After a session command fails on all connected slaves but succeeds on the
master, the servers that failed are discarded from the pool of valid
servers. If there are available servers, they will be taken into use when
the next read query is performed.
When a query is routed to an unconnected slave, it is taken into use and
the session command history is replayed. If the execution of the history
failed and there were more session commands to be executed, any queued
queries would get lost. This is due to a missing check that would detect
the fact that the server has been discarded.
