Propagate validator lists (VLs or UNLs) over the peer network:

* Whenever a node downloads a new VL, send it to all peers that
  haven't already sent or received it. It also saves it to the
  database_dir as a Json text file named "cache." plus the public key of
  the list signer. Any files that exist for public keys provided in
  [validator_list_keys] will be loaded and processed if any download
  from [validator_list_sites] fails or no [validator_list_sites] are
  configured.
* Whenever a node receives a broadcast VL message, it treats it as if
  it had downloaded it on it's own, broadcasting to other peers as
  described above.
* Because nodes normally download the VL once every 5 minutes, a single
  node downloading a VL with an updated sequence number could
  potentially propagate across a large part of a well-connected network
  before any other nodes attempt to download, decreasing the amount of
  time that different parts of the network are using different VLs.
* Send all of our current valid VLs to new peers on connection.
  This is probably the "noisiest" part of this change, but will give
  poorly connected or poorly networked nodes the best chance of syncing
  quickly. Nodes which have no http(s) access configured or available
  can get a VL with no extra effort.
* Requests on the peer port to the /vl/<pubkey> endpoint will return
  that VL in the same JSON format as is used to download now, IF the
  node trusts and has a valid instance of that VL.
* Upgrade protocol version to 2.1. VLs will only be sent to 2.1 and
  higher nodes.
* Resolves #2953

cfg/rippled-example.cfg

@@ -1052,7 +1052,7 @@
 # [crawl]
 #
 #   List of options to control what data is reported through the /crawl endpoint
-#   See https://developers.ripple.com/peer-protocol.html#peer-crawler
+#   See https://xrpl.org/peer-crawler.html
 #
 #   <flag>
 #
@@ -1093,6 +1093,16 @@
 #   counts = 0
 #   unl = 1
 #
+# [vl]
+#
+#   Options to control what data is reported through the /vl endpoint
+#   See [...]
+#
+#   enable = <flag>
+#
+#       Enable or disable access to /vl requests. Default is '1' which
+#       enables access.
+#
 #-------------------------------------------------------------------------------
 #
 # 9. Example Settings