Register validator handlers by vote extension

[mn13]

A validator should (after startup?) send a vote extension including what capabilities they can do (what future handlers they support).

The blockchain in the future might decide to assign a specific validator to a specific future (right now, the first validator introducing a result “wins”).

[warden-dev1]

Requires additional research, looks like research should be a separate big task.

[Pitasi]

• the async module should have a registry validator->capabilities

validator nodes:

• when sending a VE, it should check if its current capabilities match what's stored in the registry; and if not, it refreshes the capabilities by adding them to the VE

all the nodes:

• when processing VE they keep the registry updated with the info seen in the VEs