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/Feature.cpp

@@ -497,6 +497,7 @@ REGISTER_FEATURE(NFTokenMintOffer,              Supported::yes, VoteBehavior::De
 REGISTER_FIX    (fixReducedOffersV2,            Supported::yes, VoteBehavior::DefaultNo);
 REGISTER_FIX    (fixEnforceNFTokenTrustline,    Supported::yes, VoteBehavior::DefaultNo);
 REGISTER_FIX    (fixInnerObjTemplate2,          Supported::yes, VoteBehavior::DefaultNo);
+REGISTER_FIX    (fixNFTokenPageLinks,           Supported::yes, VoteBehavior::DefaultNo);
 // InvariantsV1_1 will be changes to Supported::yes when all the
 // invariants expected to be included under it are complete.
 REGISTER_FEATURE(InvariantsV1_1,                Supported::no, VoteBehavior::DefaultNo);

src/libxrpl/protocol/SField.cpp

@@ -113,6 +113,7 @@ CONSTRUCT_TYPED_SFIELD(sfHookStateChangeCount,  "HookStateChangeCount", UINT16,
 CONSTRUCT_TYPED_SFIELD(sfHookEmitCount,         "HookEmitCount",        UINT16,    18);
 CONSTRUCT_TYPED_SFIELD(sfHookExecutionIndex,    "HookExecutionIndex",   UINT16,    19);
 CONSTRUCT_TYPED_SFIELD(sfHookApiVersion,        "HookApiVersion",       UINT16,    20);
+CONSTRUCT_TYPED_SFIELD(sfLedgerFixType,         "LedgerFixType",        UINT16,    21);
 
 // 32-bit integers (common)
 CONSTRUCT_TYPED_SFIELD(sfNetworkID,             "NetworkID",            UINT32,     1);

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."),

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&