refactor: Use fixed size ints over (un)signed ints for serialized values

Bitcoin Core already assumes that 'unsigned int' means uint32_t and
'signed int' means int32_t. See src/compat/assumptions.h. Also, any
serialized integral value must be of a fixed size.

So make the fixed size explicit in this documenting refactor, which does
not change the behavior on any platform.

src/flatfile.h

@@ -13,16 +13,16 @@
 
 struct FlatFilePos
 {
-    int nFile{-1};
-    unsigned int nPos{0};
+    int32_t nFile{-1};
+    uint32_t nPos{0};
 
     SERIALIZE_METHODS(FlatFilePos, obj) { READWRITE(VARINT_MODE(obj.nFile, VarIntMode::NONNEGATIVE_SIGNED), VARINT(obj.nPos)); }
 
     FlatFilePos() = default;
 
-    FlatFilePos(int nFileIn, unsigned int nPosIn) :
-        nFile(nFileIn),
-        nPos(nPosIn)
+    FlatFilePos(int32_t nFileIn, uint32_t nPosIn)
+        : nFile{nFileIn},
+          nPos{nPosIn}
     {}
 
     friend bool operator==(const FlatFilePos &a, const FlatFilePos &b) {

src/index/disktxpos.h

@@ -10,14 +10,14 @@
 
 struct CDiskTxPos : public FlatFilePos
 {
-    unsigned int nTxOffset{0}; // after header
+    uint32_t nTxOffset{0}; // after header
 
     SERIALIZE_METHODS(CDiskTxPos, obj)
     {
         READWRITE(AsBase<FlatFilePos>(obj), VARINT(obj.nTxOffset));
     }
 
-    CDiskTxPos(const FlatFilePos &blockIn, unsigned int nTxOffsetIn) : FlatFilePos(blockIn.nFile, blockIn.nPos), nTxOffset(nTxOffsetIn) {
+    CDiskTxPos(const FlatFilePos& blockIn, uint32_t nTxOffsetIn) : FlatFilePos{blockIn.nFile, blockIn.nPos}, nTxOffset{nTxOffsetIn} {
     }
 
     CDiskTxPos() = default;

src/node/blockstorage.h

@@ -50,13 +50,13 @@ namespace kernel {
 class CBlockFileInfo
 {
 public:
-    unsigned int nBlocks{};      //!< number of blocks stored in file
-    unsigned int nSize{};        //!< number of used bytes of block file
-    unsigned int nUndoSize{};    //!< number of used bytes in the undo file
-    unsigned int nHeightFirst{}; //!< lowest height of block in file
-    unsigned int nHeightLast{};  //!< highest height of block in file
-    uint64_t nTimeFirst{};       //!< earliest time of block in file
-    uint64_t nTimeLast{};        //!< latest time of block in file
+    uint32_t nBlocks{};      //!< number of blocks stored in file
+    uint32_t nSize{};        //!< number of used bytes of block file
+    uint32_t nUndoSize{};    //!< number of used bytes in the undo file
+    uint32_t nHeightFirst{}; //!< lowest height of block in file
+    uint32_t nHeightLast{};  //!< highest height of block in file
+    uint64_t nTimeFirst{};   //!< earliest time of block in file
+    uint64_t nTimeLast{};    //!< latest time of block in file
 
     SERIALIZE_METHODS(CBlockFileInfo, obj)
     {