create `interface-service-discovery` based on rendezvous proposal

creating this issue here first for lack of a better place

Now that we have https://github.com/libp2p/specs/pull/44, I think we should start thinking about an additional interface for service discovery. The rational being, in the past rendezvous services (servers), provided a peer discovery mechanism, the proposal in #44 satisfies this requirements and can be adapted to be used as a pure peer discovery mechanism that implements interface-peer-discovery. However, we're lacking an interface to describe the new operations that rendezvous brings about, namely registering and discovering peers under a specific namespace. Defining an interface allows us to implement this functionality using alternative mechanisms, such as DHT.

A rendezvous service can satisfy this two use cases - pure peer discovery defined under interface-peer-discovery and service discovery interface-service-discovery.

Here is a very rough first draft (pseudo flow):

// interface-service-discovery

interface DiscoveryResponce {
  timestamp: int64;
  peers: PeerInfo[];
}

interface ServiceDiscovery {
  register(namespace: string, peer: PeerInfo): bool;
  discover(namespace?: string, limit?: int, since?: timestamp): DiscoveryResponce;
} 

I'm still not %100 sold on the naming, so take it with a grain of salt atm. The gist of the operations I want to convey with this interface can be summarized with:

• A peer wants to be able to register and be discoverable under a certain namespace
• A peer wants to discover other peers registered under a certain namespace

EDIT:

• marked discover params as optional (? means optional)
• added the word peer to in the past rendezvous services (servers), provided a...
• added DiscoveryResponce type to allow returning a timestamp along side the peer
• corrected DiscoveryResponce to return a list of peers and a timestamp

Start by reading specs PR #44 and the existing interface-peer-discovery referenced in the issue. Resolve the proposed interface's naming and design, then ensure it describes namespace-based peer registration and discovery, including the response timestamp and peer list.