Check and modify amendment blocked status with each new ledger (provided
by @wilsonianb). Honor blocked status in certain RPC commands and when
deciding whether to propose/validate.
Fixes: RIPD-1479
Fixes: RIPD-1447
Release Notes
-------------
This resolves an issue whereby an amendment blocked server would still
serve some RPC requests that are unreliable in blocked state and would
continue to publish validations.
