This replaces the previous method of attempting to clean up servers when
an unexpected exit occurs in the client (e.g. SIGINT or panic) by a
heartbeat protocol. If the server does not hear from the connecting
client within a certain interval of time (500ms in this commit), it will
de-activate itself. This prevents dangling Docker Machine server
processes from accumulating.
Signed-off-by: Nathan LeClaire <nathan.leclaire@gmail.com>
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