Break reference cycles keeping closed connections alive

[peteralm80]

Fixes #1749.

Closed connections could only be freed by the cyclic garbage collector, never
by reference counting, because of two reference cycles (details, production
numbers, and reproduction in the issue):

1. The LoggerAdapter that injects the websocket attribute into log records
   held a strong reference to the connection (all implementations).
2. In the asyncio implementation, the cancelled keepalive task retained a
   CancelledError whose traceback references the keepalive() frame and
   thus the connection.

Changes:

• New ConnectionLoggerAdapter in websockets/utils.py holds a weak
  reference to the connection and injects it into log records at logging time
  (the weakref approach suggested in Reference cycle in websockets 10? #1059). Behavior is unchanged for any
  record emitted while the connection object is alive — including "connection
  handler failed" in the server, since the handler still references the
  connection at that point — because each record receives its own strong
  reference via extra. The documented adapter-wrapping pattern
  (kwargs["extra"]["websocket"] with except KeyError) keeps working.
  Used by the asyncio, sync, and trio implementations. The deprecated legacy
  implementation is left untouched.

• Connection.connection_lost() (asyncio) dereferences keepalive_task after
  cancelling it, so the task, its CancelledError, and the pinned frame die
  by refcount once the event loop finishes the cancellation.

Tests:

• Regression tests in tests/asyncio/test_connection.py (graceful close and
  abort) and tests/sync/test_connection.py assert with gc.disable() in
  effect that a weakref to a closed connection dies — i.e. the connection is
  freed by reference counting alone. They fail without the fix and are skipped
  on non-CPython implementations.
• Unit tests for ConnectionLoggerAdapter in tests/test_utils.py.
• Two existing keepalive tests that read keepalive_task after close now
  capture the task before closing.

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