Localhost client audit concerns

[chatton]

• Replace the store.Get usage in verifyNonMembership with store.Has
• Document in verifyMembership and verifyNonMembership that the store passed is the ibc store.
• In verifyMembership and verifyNonMembership use _ as variable name for unused parameters.
• In GetTimestampAtHeight Add a comment that this function is not doing exactly the same of what it does for other client types and that it just returns the latest context timestamp.
• Create a sentinel proof for localhost and re-add validation in ValidateBasic. #3194
• Move types.LocalhostID to exported and have two variables LocalhostClientId and LocahostConnectionId.
• Add migration docs.
• Update DefaultAllowedClients godoc
• Rename getClientStateAndKVStore to getClientStateAndVerificationStore. Use clientID instead of clientState.ClientType in if check (to make sure that the whole ibc store is only returned for the sentinel localhost client).
• Use AttributeKeyConnectionId in all events in 04-channel/keeper/events

[damiannolan]

I'll pick up the sentinel proof work and create a separate PR for it.

[colin-axner]

We need to add migrations docs for the upgrade handler as well?

[damiannolan]

Addressing points 1-4 in #3199

[colin-axner]

Addressing 6, 8, 9, 10

[colin-axner]

Nice one y'all!