To support asymmetric transports, call `.listen` and `.dial` on separate
properties of the transport interface setup return value.

For symmetric transports these can be the same transport instance.

Derive the type of the components arg from a union of the internal
components plus the service map type.

Allows services to optionally define the capabilities they provide to the
rest of libp2p and also the capabilities they require from other services.

This allows, for example, the `WebRTC` transport to require the `CircuitRelay`
transport to be present, or `KAD-DHT` (or anything that uses a topology)
to require the identify protocol.

Fixes #2263
Refs #2135

Writes up the perceived wisdom on service implementation.

Remove code left in by accident.

Chrome limits how many RTCPeerConnections a given tab can instantiated
during it's lifetime - https://issues.chromium.org/issues/41378764

To delay hitting this limit, only create the dial-end RTCPeerConnection
once a relayed connection has successfully been opened to the dial
target, this prevents needlessly creating RTCPeerConnections when the
dial fails before they are actually used.

Fixes #2591

In order to allow refreshing STUN/TURN credentials between dials,
allow passing a function that returns rtc config.

Fixes #2554