Increase validation quorum to 80%

All listed validators are trusted and quorum is 80% of trusted
validators regardless of the number of:
* configured published lists
* listed or trusted validators
* recently seen validators

Exceptions:
* A listed validator whose master key has been revoked is not trusted
* Custom minimum quorum (specified with --quorum in the command line)
  is used if the normal quorum appears unreachable based on the number
  of recently received validators.

RIPD-1640

cfg/validators-example.txt

@@ -47,6 +47,12 @@
 
 # The default validator list publishers that the rippled instance
 # trusts.
+#
+# WARNING: Changing these values can cause your rippled instance to see a
+#          validated ledger that contradicts other rippled instances'
+#          validated ledgers (aka a ledger fork) if your validator list(s)
+#          do not sufficiently overlap with the list(s) used by others.
+#          See: https://arxiv.org/pdf/1802.07242.pdf
 
 [validator_list_sites]
 https://vl.ripple.com