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/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)
     {