The 'validation_seed' RPC command was used to change the validation
key used by a validator at runtime.
Its implementation was commented out with commit fa796a2eb5
which has been included in the codebase since the 0.30.0 release
and there are no plans to reintroduce the functionality at this
point.
Validator operators should migrate to using validator manifests
instead.
This fixes#2748.
In support of dynamic validator list, this changeset:
1. Adds a new `validator_list_expires` field to `server_info` that
indicates when the current validator list will become stale.
2. Adds a new admin only `validator_lists` RPC that returns the
current list of known validators and the most recent published validator
lists.
3. Adds a new admin only `validator_sites` RPC that returns the list of
configured validator publisher sites and when they were most recently
queried.
