ICS28: Remove existing pending spawn/stop proposals when adding new ones for a given consumer chain ID

[danwt]

_ See implementation mirror issue cosmos/interchain-security#155 _

When a proposal gets handled it will be added to a pending list if spawnTime has not yet passed.

ibc/spec/app/ics-028-cross-chain-validation/methods.md

Lines 351 to 361 in ad30596

	// PCF: Provider Chain Function
	// implements governance proposal Handler
	function SpawnConsumerChainProposalHandler(p: SpawnConsumerChainProposal) {
	if currentTimestamp() > p.spawnTime {
	CreateConsumerClient(p)
	}
	else {
	// store the proposal as a pending spawn proposal
	pendingSpawnProposals.Append(p)
	}
	}

I'm suspicious of what might happen if we allow multiple entries in pending for a given chain. I cannot think of any situation where this would be beneficial. It could only lead to strange behavior.

I think we should ensure that SetPendingClientInfo only has at most one entry per chainID and updates the existing entry if it already exists instead of creating a new entry.