Per-connection protocol list

Nobody has claimed this yet.

Currently, identify operates at a _peer_ level. However, this doesn't match up with reality. In practice, there are some protocols we only want to speak over connections with specific properties. Mostly with respect to transient connections, but we may end up having, e.g., network, interface, or even IP-based restrictions.

To solve this, we should:

1. Run identify on all connections (we already do this).
2. Associate the protocol list with the connection itself (see https://github.com/libp2p/go-libp2p/issues/2355).
3. When creating a new stream, find the best connection that supports the given protocol.
4. Whenever a connection is created/destroyed, emit a "protocol change" event describing the connection-specific protocols lost/gained.
5. Provide a way to set a protocol handler with some set of constraints. Ideally an arbitrary function that's called on each new connection (during identify) and on a stream's connection before the handler is invoked. That shouldn't be that slow and it's _really_ flexible.

Why? Bitswap, for example only wants non-transient connections and it doesn't even want to hear about peers with transient-only connections. Honestly, this applies to pretty much all of our protocols except the DHT and hole-punching protocols.

Start by reading the existing identify flow and stream-creation path, then review the related issue #2355 for connection-level protocol association. Trace how connections are created and destroyed and how protocol handlers are registered. Done means protocols can be selected per connection, connection-specific protocol changes are emitted, and handlers can apply connection constraints.

Written by the indexing model from the issue text.