This rewrites the shutdown logic to simplify the shutdown signalling.
All cleanup is now run from deferred functions in the main function and
channels are used to signal shutdown either from OS signals or from
other subsystems such as the RPC server and windows service controller.
The RPC server has been modified to use a new channel for signalling
shutdown that is exposed via the RequestedProcessShutdown function
instead of directly calling Stop on the server as it previously did.
Finally, it adds a few checks for early termination during the main
start sequence so the process can be stopped without starting all the
subsystems if desired.
This is a backport of the equivalent logic from Decred with a few slight
modifications. Credits go to @jrick.
When an OS reboots or shuts down, it sends all processes SIGTERM before
sending SIGKILL. This allows btcd to do a proper shutdown which most
importantly closes the database.
