* Only require adding the new feature names in one place. (Also need to
increment a counter, but a check on startup will catch that.)
* Allows rippled to have the code to support a given amendment, but
not vote for it by default. This allows the amendment to be enabled in
a future version without necessarily amendment blocking these older
versions.
* The default vote is carried with the amendment name in the list of
supported amendments.
* The amendment table is constructed with the amendment and default
vote.
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