reworked the reference count mechanism on the session. Introduced a
FREE state for the session and alter the session destruction flow so
that we only remove the session when all the DCB's have singled they
have finished processing events for the DCB rather than when the first
thread decides to clsoe the DCB.
routeQuery()
(dcb->func).close(dcb);
This will prevent setting again dcb->sate = DCB_STATE_POLLING in return_rc label.
The log entry for
[gw_read_client_event] ioctl FIONREAD for fd 21 failed. errno 9, Bad file descriptor. dcb->state = 3
now is not present anymore.
