Document layering of IBC modules

[mhofman]

I'm having a hard time wrapping my head around the different IBC flows, and which steps each IBC module goes through.

I would like to see a flow diagram of the different modules / middleware (pfm, vibc, vtransfer, address hooks, etc.) and their functions involved when processing IBC messages (send, recv, ack, timeout), in particular for IBC transfers under the different supported flows (regular, pfm, hooked address, ...).