Add support for Packet Delay (on connection)

[ethanfrey]

This one is tricky. We need to allow the option when creating a connection, and read the option when using an existing connection.

If delay=0, then the general behavior we have everywhere works.

If delay > 0, then we must do the following to relay packets.

• SendPacket available at height H on chain A
• Submit A header of H+1 on chain B (to prove H)
• Store proof of packet pointing to this header (H+1)
• Wait delay seconds after update client
• Submit the SendPacket proof pointing to header (H+1), not a newer header

This means there is a delay between updating a client and being able to use that client header to prove anything. As a potential protection against byzantine networks.