Use int conversion for Option class

The current implementation generates warnings on MSVC. However, we have
no real use cases for double-typed UCI option values now. Also parameter
tuning only accepts following three types:

  int, Value, Score

closes https://github.com/official-stockfish/Stockfish/pull/4505

No functional change

src/tt.cpp

@@ -87,7 +87,7 @@ void TranspositionTable::clear() {
 
   std::vector<std::thread> threads;
 
-  for (size_t idx = 0; idx < Options["Threads"]; ++idx)
+  for (size_t idx = 0; idx < size_t(Options["Threads"]); ++idx)
   {
       threads.emplace_back([this, idx]() {
 
@@ -98,7 +98,7 @@ void TranspositionTable::clear() {
           // Each thread will zero its part of the hash table
           const size_t stride = size_t(clusterCount / Options["Threads"]),
                        start  = size_t(stride * idx),
-                       len    = idx != Options["Threads"] - 1 ?
+                       len    = idx != size_t(Options["Threads"]) - 1 ?
                                 stride : clusterCount - start;
 
           std::memset(&table[start], 0, len * sizeof(Cluster));