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fixes #7

It could have been sitting around for a while.

add tests for the listener

fix off-by-one error in the threshold

Forgot to initialize it. For some absurd reason, gx will just auto-initialize to
go-ipfs...

wait after accepting a new connection if the queue is full

keep accepting and negotiating connections until we have 16 ready and queued up

Also, add a context canceled check to the loop just in case (accept could, if
buggy, return temporary errors after it's closed).

Otherwise, we'll have an annoying saw-tooth pattern where we'll accept a bunch
of connections, set them up in parallel, and then wait until we pass them all
back up to the swarm before accepting any more.

This commit allows us to queue up 16 ready connections before ceasing to accept
any new connections. We'll still probably have a saw-tooth pattern under heavy
load but it should be less pronounced (and we can improve the situation by
upping the queue size.