refactor: Use uint64_t over size_t for serialized-size values

The values are small enough to fit in size_t, but to avoid having to
think about it, just use uint64_t consistently for all architectures.

On 64-bit systems, this refactor is a no-op. On 32-bit systems, it could
avoid bugs in the theoretical and unexpected case where a 32-bit size_t
is too small and overflows.

src/test/flatfile_tests.cpp

@@ -39,8 +39,8 @@ BOOST_AUTO_TEST_CASE(flatfile_open)
     std::string line2("Digital signatures provide part of the solution, but the main benefits are "
                       "lost if a trusted third party is still required to prevent double-spending.");
 
-    size_t pos1 = 0;
-    size_t pos2 = pos1 + GetSerializeSize(line1);
+    uint64_t pos1{0};
+    uint64_t pos2{pos1 + GetSerializeSize(line1)};
 
     // Write first line to file.
     {