Avoid using numeric_limits for sequence numbers and lock times

Switches to named constants, because numeric_limits calls can be harder to read
and less portable.

Change was suggested by James O'Beirne <james.obeirne@gmail.com> in
https://github.com/bitcoin/bitcoin/pull/10973#discussion_r213473620

There are no changes in behavior except on some platforms we don't support
(ILP64, IP16L32, I16LP32), where SignalsOptInRBF() and MutateTxAddInput()
functions would now work correctly.

src/bitcoin-tx.cpp

@@ -255,7 +255,7 @@ static void MutateTxAddInput(CMutableTransaction& tx, const std::string& strInpu
         throw std::runtime_error("invalid TX input vout '" + strVout + "'");
 
     // extract the optional sequence number
-    uint32_t nSequenceIn=std::numeric_limits<unsigned int>::max();
+    uint32_t nSequenceIn = CTxIn::SEQUENCE_FINAL;
     if (vStrInputParts.size() > 2)
         nSequenceIn = std::stoul(vStrInputParts[2]);
 

src/rpc/rawtransaction.cpp

@@ -346,7 +346,7 @@ CMutableTransaction ConstructTransaction(const UniValue& inputs_in, const UniVal
 
     if (!locktime.isNull()) {
         int64_t nLockTime = locktime.get_int64();
-        if (nLockTime < 0 || nLockTime > std::numeric_limits<uint32_t>::max())
+        if (nLockTime < 0 || nLockTime > LOCKTIME_MAX)
             throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid parameter, locktime out of range");
         rawTx.nLockTime = nLockTime;
     }
@@ -368,18 +368,18 @@ CMutableTransaction ConstructTransaction(const UniValue& inputs_in, const UniVal
 
         uint32_t nSequence;
         if (rbfOptIn) {
-            nSequence = MAX_BIP125_RBF_SEQUENCE;
+            nSequence = MAX_BIP125_RBF_SEQUENCE; /* CTxIn::SEQUENCE_FINAL - 2 */
         } else if (rawTx.nLockTime) {
-            nSequence = std::numeric_limits<uint32_t>::max() - 1;
+            nSequence = CTxIn::SEQUENCE_FINAL - 1;
         } else {
-            nSequence = std::numeric_limits<uint32_t>::max();
+            nSequence = CTxIn::SEQUENCE_FINAL;
         }
 
         // set the sequence number if passed in the parameters object
         const UniValue& sequenceObj = find_value(o, "sequence");
         if (sequenceObj.isNum()) {
             int64_t seqNr64 = sequenceObj.get_int64();
-            if (seqNr64 < 0 || seqNr64 > std::numeric_limits<uint32_t>::max()) {
+            if (seqNr64 < 0 || seqNr64 > CTxIn::SEQUENCE_FINAL) {
                 throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid parameter, sequence number is out of range");
             } else {
                 nSequence = (uint32_t)seqNr64;

src/script/script.h

@@ -38,6 +38,12 @@ static const int MAX_STACK_SIZE = 1000;
 // otherwise as UNIX timestamp.
 static const unsigned int LOCKTIME_THRESHOLD = 500000000; // Tue Nov  5 00:53:20 1985 UTC
 
+// Maximum nLockTime. Since a lock time indicates the last invalid timestamp, a
+// transaction with this lock time will never be valid unless lock time
+// checking is disabled (by setting all input sequence numbers to
+// SEQUENCE_FINAL).
+static const uint32_t LOCKTIME_MAX = 0xFFFFFFFFU;
+
 template <typename T>
 std::vector<unsigned char> ToByteVector(const T& in)
 {