quic: use an enum for sentPacket state

The sentPacket type tracks the state of a packet sent to the peer.
Packets can be in progress, acknowledged, or lost.
Track this state with an enum rather than a set of bools,
to avoid the possibility of nonsensical states such as a packet
being both acknowledged and lost.

This also simplifies a following change to add an "unsent" state
for intentionally skipped packet numbers.

Change-Id: I87c8fc399c72337c033ab7ec5ec8db2c56c732f9
Reviewed-on: https://go-review.googlesource.com/c/net/+/664297
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>