Multiple signatures for same data in anchoring transactions

[slowli]

The MsgAnchoringSignature transaction implementation effectively allows a single validator to submit an arbitrary number of signatures for the same anchoring transaction. This is because it is possible to create different valid signatures over the same bitcoin transaction and input index/pubkey.

The bug may probably lead to various exploits.

Solution sketch: do not accept more than one signature from the same validator in MsgAnchoringSignature.