Previously, we relied on the owner of the handshake future to drive it to
completion. This meant that there were cases where handshakes might never be
completed, just because nothing was actively polling them.
The previous outbound peer connection logic got requests to connect to new
peers and processed them one at a time, making single connection attempts
and retrying if the connection attempt failed. This was quite slow, because
many connections fail, and we have to wait for timeouts. Instead, this logic
connects to new peers concurrently (up to 50 at a time).
This means that all sub-modules of `peer` can import everything they need from
the `peer` module itself, without having to be aware of the internal structure
of their sibling modules.
It's only responsible for doing the handshakes, so it should be named that way,
and then we can have a Connector responsible for actually opening the TCP
connection.
Henry de Valence