Address rare corruption of NFTokenPage linked list (#4945)

* 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.

src/libxrpl/protocol/TER.cpp

@@ -137,6 +137,7 @@ transResults()
         MAKE_ERROR(tefTOO_BIG,                     "Transaction affects too many items."),
         MAKE_ERROR(tefNO_TICKET,                   "Ticket is not in ledger."),
         MAKE_ERROR(tefNFTOKEN_IS_NOT_TRANSFERABLE, "The specified NFToken is not transferable."),
+        MAKE_ERROR(tefINVALID_LEDGER_FIX_TYPE,     "The LedgerFixType field has an invalid value."),
 
         MAKE_ERROR(telLOCAL_ERROR,            "Local failure."),
         MAKE_ERROR(telBAD_DOMAIN,             "Domain too long."),