quic: don't re-lose packets when discarding keys

When discarding keys for a packet number space, we remove any
in-flight packets in that space from our count of bytes in flight.
Skip any packets in the space that were already acked or declared lost.

Also skip over acked/lost packets when discarding state in response
to a Reset message. This change shouldn't have any effect, since we
never declare packets acked/lost until we receive an ACK, and we
never handle a Reset after receiving an ACK.

Change-Id: I8842f03aa06e57a834a211f1284a48fe5d469db3
Reviewed-on: https://go-review.googlesource.com/c/net/+/664335
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Auto-Submit: Damien Neil <dneil@google.com>