ackhandler.Packet pool is mostly ineffective

[marten-seemann]

The ackhandler.Packet pool introduced in #3525 seems to be leaky.

Instrumenting the code with simple counters for the Get and Put operations shows that we're getting a lot more buffers than we're putting back. On the receiving end of a 1 GB transfer, we get the following:

get: 228214, put: 11783

230k Gets seem about right, it means we're sending an ACK for every 3-4 packets we receive. I would've expected most of these packets to be acknowledged at some point though, which would release them back into the pool.

[marten-seemann]

We don't save non-ack-eliciting packets to our packet history, so we're leaking all the Packets there.