app, queue: generalize handler and move to the queue package
queue: added Preempt function

Preempt stops all running handlers.
queue: fix data race in connection handling
queue: change Preempt so it copy the map

It already used another reference for avoiding a dead lock, but there
was still a race.
queue: added cleanQ function for use in tests

Just moved from SetUpSuite to an isolated function.
queue: fix bug that prevent Handler from stoping

In the old code, first the handler asked for messages in the queue, and
if it failed to get a message from the queue, it checked if the handler
state is "running". That means that even when we call handler.Stop(), it
would stop only there was no message left in the queue.

Now it checks the state of the handler before trying to reserve more
jobs from the queue.
api, app: preempt handlers on TearDownSuite
Makefile: don't run tests concurrently

That's it. I give up.
Makefile: omit command on target race