Specify exact scan/query/submit relayer algorithm

[cwgoes]

Follow-up from #166

[cwgoes]

Should cover:

• Handshakes (through relayer module)
• Outgoing packets (then calling recvPacket through relayer module)

Basic algorithm:

• Between two chains
• Scan state, check for sendable datagrams
• Interleave client updates as necessary for updated state commitments
• Valid datagrams depend on state of both chains

[cwgoes]

Note: this can only work generically with implementations which use the relayer module, since otherwise the relayer process would need knowledge of module-specific message encoding details to determine how to invoke recvPacket.

[cwgoes]

For simplicity we'll always update the client in the example algorithm.