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/TxFormats.cpp

@@ -505,6 +505,14 @@ TxFormats::TxFormats()
             {sfOracleDocumentID, soeREQUIRED},
         },
         commonFields);
+
+    add(jss::LedgerStateFix,
+        ttLEDGER_STATE_FIX,
+        {
+            {sfLedgerFixType, soeREQUIRED},
+            {sfOwner, soeOPTIONAL},
+        },
+        commonFields);
 }
 
 TxFormats const&