* Update multiple deps
  * websocket (bug fixes)
  * sha256-simd (arm build fix)
  * mdns (race conditions)

* Update for go-multiaddr protocol definition refactor. Multiaddr protocols have
been moved out of libp2p transports into go-multiaddr itself.
* Fix some issues with non-constant time operations involving libp2p secret keys.

* Add a `Filters` option to the constructor for specifying an address filter.
This can be used to restrict which peers libp2p will connect to by IP/port/transport.

* Add a `UserAgent` option to libp2p for specifying the libp2p user-agent.
Previously, this was specified using a global `ClientVersion` global variable.

* Set the default user-agent to `$repo@$version` (where `$repo` is the
_downstream_ project importing go-libp2p).