Add requestMessageReconfirm to HomeAMB

[k1rill-fedoseev]

Adds functionality requested under #604 . This PR only makes changes relevant to the AMB contracts. Changes for erc-to-native bridge will be implemented later, in a distinct PR.

For AMB, the requested functionality appeared to be easily integrated to the existing codebase. No oracle modifications are necessary at all. Emitting duplicate UserRequestForSignature and CollectedSignatures events is safe, since oracle does not use transaction hash for constructing the signed message.

The flow might be the following:

• User finds out that current set of signatures is invalid.
• User calls requestMessageReconfirm function which re-emits UserRequestForSignature and prunes all invalid signatures associated with this message.
• Oracles catch the UserRequestForSignature event and process it in the usual way. Oracles that have not already submitted their signatures, sign a message and call submitSignature.
• Once enough valid signatures are collected, bridge contract re-emits CollectedSignatures event.
• If necessary, oracle catch CollectedSignatures event and executes a message.