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The global per-peer dial timeout had a significant drawback: When dialing many
peers, this timeout could cause libp2p to cancel dials while they were still
stuck in the limiter.

A better but more complicated approach is a time budget system but we can
implement that later.

This change simply applies the limit to each `DialPeer`/`NewStream` call
independently and makes it easy to override. While old timeout tried to
account for how much we're willing to spend dialing a single peer, this new
timeout tries to account for the amount of time a single "client" is willing to
wait for a dial to complete before they no longer care.

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.

We already setup the connection from within the new connection handler. No need
to do it *again* on dial.

It doesn't exist anymore. This is now handled entirely though contexts.

fixes libp2p/go-libp2p-kad-dht#96