Replace /ack API with proposals

[eddyashton]

We believe the existing /ack paths can be removed, and instead done in the proposal flow. Roughly:

• Remove any consideration of member activeness from the C++ code
• Add a new ack action, which requires no votes and always passes resolve, and updates the member's state to Active
• When tallying votes in the default resolve() function, only count members if they are Active

We could also make submit any proposal an implicit ack, so that the proposer doesn't need to submit an ack action directly.
This reduces the API surface, and allows services to opt out of the ACK behaviour entirely if they wish, by removing it from their constitutions.

[achamayou]

Note: we need to update propose to allow proposals from inactive members.

Also note: the reason ack() can be empty is because the proposal id includes the root of the merkle tree, and is effectively equivalent to the pre-existing, explicit ack endorsement.