Limit max size of packets

[gjermundgaraba]

Version, encoding and payload currently have no size limits. This could lead to very large transactions if a chain does not have a very restrictive global limits (most don't) if abused. This should ideally been solved on the SDK level, but the current global limits there are very high, and we can't rely on those for the time being.

Points that needs discussion:

• Do we want/need a limit on packet size?
• Is "global" packet size the best place to add such a limit?
• What should the limit be?
• If we do this, should it only be for V2 packets?

[DimitrisJim]

Ref relevant spec issue cosmos/ibc#1160

(though spec would in theory only mention implementations could opt to add limits, so hashing out what these look like is ibc-go level discussion)