fix: stabilize voting threshold for amendment majority mechanism (#4410)

Amendment "flapping" (an amendment repeatedly gaining and losing
majority) usually occurs when an amendment is on the verge of gaining
majority, and a validator not in favor of the amendment goes offline or
loses sync. This fix makes two changes:

1. The number of validators in the UNL determines the threshold required
   for an amendment to gain majority.
2. The AmendmentTable keeps a record of the most recent Amendment vote
   received from each trusted validator (and, with `trustChanged`, stays
   up-to-date when the set of trusted validators changes). If no
   validation arrives from a given validator, then the AmendmentTable
   assumes that the previously-received vote has not changed.

In other words, when missing an `STValidation` from a remote validator,
each server now uses the last vote seen. There is a 24 hour timeout for
recorded validator votes.

These changes do not require an amendment because they do not impact
transaction processing, but only the threshold at which each individual
validator decides to propose an EnableAmendment pseudo-transaction.

Fix #4350

src/ripple/app/main/Application.cpp

@@ -1333,6 +1333,9 @@ ApplicationImp::setup(boost::program_options::variables_map const& cmdline)
                 << "Invalid entry in [" << SECTION_VALIDATOR_LIST_SITES << "]";
             return false;
         }
+
+        // Tell the AmendmentTable who the trusted validators are.
+        m_amendmentTable->trustChanged(validators_->getQuorumKeys().second);
     }
     //----------------------------------------------------------------------
     //