TableProbe: make solver_found() never return

The dispatcher thread doesn't use the solver thread value, add a call
to Thread::exit() and remove misleading returns.
TableProbe: factor solver_send_expect() out

Make 'found' a synchronous message and reuse solver_send_expect() in
both solver_found() and solver_fits().
TableProbe: report preliminary flow loading rate

Update dispatcher_callback() to report flow loading rate only on solver
request. Update solver_thread() to report the rate once before
proceeding with the bisection and once after.
TableProbe: do not measure rate of failed rounds

barrier_reply() would refine the measure regardless if the test round
was 100% successful. Fix it to consider only successful rounds.
TableProbe: rename FSM states for clarity
TableProbe: improve rate measuring accuracy

The time spent deleting flows from a switch is a notional fraction of
the overall testing time, because Trema always sets the
OFPFF_SEND_FLOW_REM flag to keep its internal mapping working, hence
OFPT_FLOW_REMOVED messages always consume time and bandwidth on wire
making it from the switch, even if not delivered to the controller
class.

Introduce a new "clearing" FSM state that deletes all flows before
handing the emptied switch over to "installing" state, which proceeds
as before. This way the measured rate actually stands for loading.