Amendment flapping usually occurs when an amendment is on
the verge of gaining majority, and a validator not in favor
of the amendment goes offline.

If fixAmendmentFlapping activates then two changes occur:

1. The number of validators in the UNL determines the required
threshold for an amendment to gain majority.

2. The AmendmentTable keeps a record of the most recent
Amendment vote received from each trusted validator in the
UNL.  If no validation arrives from a given validator, then
the AmendmentTable assumes that the previously received vote
has not changed.

Fixes XRPLF#4350