Application termination

[ForNeVeR]

We should investigate how to make Emulsion to shut down gracefully, to not lose any messages if possible.

First of all: on termination, we should cancel the root cancellation token. Currently, we don't.

When shutting down, Emulsion should stop processing new messages (or put them into a database if available, see also #24), then try to send the ones already in the buffer, and only then stop completely.