ICS28: Replace "Initiator" w/ "Caller" and “Trigger Event”

[mpoke]

In the technical specification of CCV (see https://github.com/cosmos/ibc/blob/marius/ccv/spec/app/ics-028-cross-chain-validation/technical_specification.md), the subprotocols are specified by describing every method. This entails the pseudocode followed by the following fields:

• Initiator
• Expected precondition
• Expected postcondition
• Error condition

To remove ambiguity, these fields should be restructured, i.e.,

• Caller The entity invoking the method (e.g., another ABCI module). This is useful to keep track of the interfaces with other modules.
• Trigger Event The event triggering the call to this method (e.g., a message is received). This is useful to reason about the overall logic of the protocol.
• Precondition The internal state that enables the call of this method, i.e., so called "guards" in TLA+.
• Postcondition The modifications to the internal state as a result of executing this method. This should also include abortTransactionUnless calls (i.e., reverting a transaction).
• Error Condition The errors triggered as a result of executing this method. This consists of abortSystemUnless calls.