GitLab

Modify our two transport lookup functions to log loudly when there are no
transports registered.

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.