Powerhose

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- auto-unregistring of dead/slow workers

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- introduced a graceful shutdown
- added a worker registering so restarting workers are not
impacting the system

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- drastically reduced the number of used FDs, mainly by
reusing the same context when possible

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- the broker exits if there's already a valid broker running
in the socket.
- powerhose-broker gained 2 new options: --check and --purge-ghosts

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- implemented timeout_max_overflow in the client.
- the stacks are dumped on worker timeouts
- now using delayed callbacks for the heartbeat