* Add fixNFTokenPageLinks amendment:
It was discovered that under rare circumstances the links between
NFTokenPages could be removed. If this happens, then the
account_objects and account_nfts RPC commands under-report the
NFTokens owned by an account.
The fixNFTokenPageLinks amendment does the following to address
the problem:
- It fixes the underlying problem so no further broken links
should be created.
- It adds Invariants so, if such damage were introduced in the
future, an invariant would stop it.
- It adds a new FixLedgerState transaction that repairs
directories that were damaged in this fashion.
- It adds unit tests for all of it.
