ADD: ErrataTx

Nodes can send in an ErrataTx

type errataTx struct {
	ID          TxID    `json:"id"`
	Chain       Chain   `json:"chain"`
}

Once consensus is achieved, THORChain follow this logic:

1. If Swap, then decrement pool balance by the amount of the original tx in internal state.
2. If Stake, then decrement stakeUnits of the associated staker by the amount of the original tx in internal state. Then decrement pool balance by the amount of the original tx in internal state.

On the ObservedTx struct, add "removed" and null out OutHashes.

type ObservedTx struct {
	Tx             common.Tx        `json:"tx"`
	Status         status           "removed"
	OutHashes      common.TxIDs     "" 
	BlockHeight    int64            `json:"block_height"`
	Signers        []sdk.AccAddress `json:"signers"` 
	ObservedPubKey common.PubKey    `json:"observed_pub_key"`
}