Make AsyncContext visible from TransportObserver#onNewConnection()

[idelpivnitskiy]

Motivation:

Users who want to correlate TransportObserver#onNewConnection() events with
the request object may use AsyncContext to propagate correlation key.
However, AsyncContext is not visible from TransportObserver#onNewConnection()
because it runs from IO-thread. It's also important to invoke
TransportObserver#onNewConnection() callback earlier on the client-side.

Modifications:

• Trigger TransportObserver#onNewConnection() as soon as someone subscribes to
  TcpConnector#connect single on the client-side to get access to the caller's
  thread AsyncContext;
• Trigger TransportObserver#onNewConnection() at the beginning of initChannel
  on the server-side for consistency with the client-side;
• Update internal classes and tests to use new API;
• Add tests to verify AsyncContext is visible for expected callbacks;

Result:

AsyncContext is visible from TransportObserver#onNewConnection() on the
client-side.

[idelpivnitskiy]

In some cases, ConnectionObserver events are generated by network events when the request-related AsyncContext is unknown. For those cases, users should capture the AsyncContext state from other callbacks, keep it in local variable(s), and propagate when necessary. Examples:

1. ConnectionObserver#onSecurityHandshake() and all SecurityHandshakeObserver callbacks should use the state captured in TransportObserver#onNewConnection().
2. ReadObserver/WriteObserver events should use the state captured in DataObserver callbacks.
3. StreamObserver#streamClosed events should use the state captured in StreamObserver#streamEstablished() or MultiplexedObserver#onNewStream().
4. connectionClosed events can be correlated with request(s) using requestId<->connectionId correlation at the higher levels.

@Scottmitch we can cover more cases (for example, onSecurityHandshake ) using another technique if necessary. Because we have access to subscriber at the time we invoke this callback, we can check if it's an instance of ContextPreservingSubscriber and extract the AsyncContextMap from it. However, it will require some movements for ContextPreserving* classes. I will defer that until we have a use-case.

[Scottmitch]

agreed lets defer until necessary.

[Scottmitch]

agreed lets defer until necessary.