fix defunct processes and deregister server before it stops

[chaosue]

as shown above, after stopping the master cells process, there might be some sub processes left defunct in random.
two points in cells may lead to this problem.

1. The Stop method in main routine does not wait the StartAndWait method to complete waiting its sub process.
2. The gateway.data plugin , which uses the minio package, does not fork, so it still serves in the main process, while the minio's exit function will directly end the main process when stopping the pulgin. However the minio's exit function cannot be customized, so there's no way to tweak this behaviour from the minio side at present. Only the forking way may resolve it.

so whether there will be defunct processes depends on who (the main process and its sub processes) exits first in random.

another point, shall we deregister the server to notify the client-ends in advance before stopping the server to prevent unexpected future requests.

[cdujeu]

needs more attention, we will dig that one, seems to have side-effect (require multiple Ctrl+C sometimes).
Also @chaosue for the next ones, make sure to not mess up the imports : we always group them in three blocks: standard lib, third party libs, cells modules.

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