Keep the signed merkle root of the Ethereum bridge pool updated

[batconjurer]

This will have two implementations: one for vote extensions and one for without.

For vote extensions:

• Each validator signs the latest root and adds to their vote extensions
• At every block, these signatures are aggregated and included in the block proposal to be included in storage.

Without vote extensions:

• Each validator signs the latest root and broadcasts it as a protocol tx
• These signatures appear on chain and we keep a tally of partial proofs of different merkle tree roots in storage.
• We maintain the latest root which has reached a quorum of signatures.

[cwgoes]

@batconjurer is this up-to-date?