Make exception/signal handlers idempotent

Make exception/signal handlers idempotent

Calling asyncio.Future.set_exception() or asyncio.Future.set_result()
more than once for a given Future object raises an
asyncio.InvalidStateError exception.

In the case of AsyncServer:

  - it is enough to capture the first exception raised by higher-level
    logic as no exceptions at all are expected to be raised in the first
    place,

  - no distinction is made between SIGINT and SIGTERM; the only purpose
    of the signal handler is to make the server exit cleanly.

Given the above, make both AsyncServer._handle_exception() and
AsyncServer._signal_done() idempotent by ignoring
asyncio.InvalidStateError exceptions raised by the relevant
asyncio.Future.set_*() calls.