MEDIUM: h2: apply a timeout to h2 connections

MEDIUM: h2: apply a timeout to h2 connections

Till now there was no way to deal with a dead H2 connection. Now each
connection creates a task that wakes up to kill the connection. Its
timeout is constantly refreshed when there's some activity. In case
the timeout triggers, the best effort attempts are made at sending a
clean GOAWAY message before closing and signaling the streams.

The timeout is automatically disabled when there's an active stream on
the connection, and restarted when the last stream finishes. This way
it should not affect long sessions.