Removes:
* go-libp2p-conn logic (moved to transports)
* go-libp2p-peerstream (moved here)
Changes:
* New transport interface.
* Explicit over implicit (the implicit automagic now all lives in go-libp2p):
* No more default transports, muxers, etc.
* No more fallback dialer. Transports are now *required*.
* Stream opening:
* Connection picking logic (instead of just picking the first).
* Tries harder to open a stream of some connections happen to be closed.
* Stream closing:
* No longer treats half-closed streams as fully closed. Users *must* read the
an EOF or reset the stream for it to be garbage collected.
* No more polling for dead connections.
* Multiplexers are now *non-optional*. Really, they haven't been optional for a
while but we still pretended that they were.
* No more Network type alias. It added a bunch of code and didn't really provide
anything but an alternative set of methods that do the same thing.
* Notifications:
* New guarantee: connection open notifications will complete before connection
close notifications begin.
* Given that, notifications are now delivered in parallel. No more
notification backlogs blocking connection closing/opening.
