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.
Manifests of validators newly added to a published validator list are
not reliably propagated to network nodes.
This solves the problem by allowing a published validator list to
include the manifest.
RIPD-1559
Validator lists from configured remote sites are fetched at a regular
interval. Fetched lists are expected to be in JSON format and contain the
following fields:
* "manifest": Base64-encoded serialization of a manifest containing the
validator publisher's master and signing public keys.
* "blob": Base64-encoded JSON string containing a "sequence",
"expiration" and "validators" field. "expiration" contains the Ripple
timestamp (seconds since January 1st, 2000 (00:00 UTC)) for when the
list expires. "validators" contains an array of objects with a
"validation_public_key" field.
* "signature": Hex-encoded signature of the blob using the publisher's
signing key.
* "version": 1
* "refreshInterval" (optional)
