* Adds local file:// URL support to the [validator_list_sites] stanza.
The file:// URL must not contain a hostname. Allows a rippled node
operator to "sideload" a new list if their node is unable to reach
a validator list's web site before an old list expires. Lists
loaded from a file will be validated in the same way a downloaded
list is validated.
* Generalize file/dir "guards" from Config test so they can be reused
in other tests.
* Check for error when reading validators.txt. Saves some parsing and
checking of an empty string, and will give a more meaningful error.
* Completes RIPD-1674.
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)
Instead of specifying a static list of trusted validators in the config
or validators file, the configuration can now include trusted validator
list publisher keys.
The trusted validator list and quorum are now reset each consensus
round using the latest validator lists and the list of recent
validations seen. The minimum validation quorum is now only
configurable via the command line.
* This fixes an uncommon, but annoying, spurious failure running this
test, particularly in release builds. This appears to be an issue with
Windows of the FS where quickly creating and deleting the same
directory repeatedly will eventually fail.
* RIPD-1390
