quic: avoid busy-loop when keep-alive is blocked by congestion control

When a connection's keep-alive timer expires, we send a PING frame.
Sending an ack-eliciting frame such as a PING clears the idle timer.
However, if the frame could not be sent (due to being blocked by
congestion control, for example), the keep-alive timer was not cleared
resulting in the connection event loop busy-waiting on the timer.

Now, when the keep-alive timer expires we clear it immediately and
set a flag indicating that we want to send a keep-alive.

Minor related fix: Disable the keep-alive and idle timers when
entering the closing/draining states. (We never set these timers
when in these states, but we failed to clear them on entry.)

Change-Id: I9f3f2327f2138273b7e10c4185cf9c176a6a6964
Reviewed-on: https://go-review.googlesource.com/c/net/+/801882
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Nicholas Husin <husin@google.com>
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Nicholas Husin <nsh@golang.org>