* ackhandler: simplify deletion of old packets in packet history
* ackhandler: use a slice to keep track of sent packets
This is a vastly simpler data structure than the combination of map
and linked list used before. It avoids using a linked list (bad cache
locality) and a sync.Pool (for list elements), as well as having to do
hash table lookups.
In the future, this can be easily replaces with a ring buffer, avoiding
all allocations.
* ackhandler: don't store packets that were declared lost
Especially the sentPacketHistory linked list shows up in allocation
profiles, since a new list element is allocated for every single packet
we send.
Using a pool for the receiving path, as well as for the frame sorter, is
less critical, since we're tracking ranges there instead of individual
packets / frames, but it doesn't hurt either.
The other occurrences where we use a linked list (connection ID tracking
and the token store) are used so rarely (a few times over the lifetime
of the connection) that using a pool doesn't make any sense there.
* optimize FirstOutstanding
* fix variable naming
* bug fix
* minor code improvements
* add a test to make sure that `Iterate` iterates in the right order
* add comment
Packet numbers now start at 0 for every packet number spaces.
Furthermore, an ACK frame now only acknowledges packets in the packet
number space it was sent in.
