change conns_in_use_ in connection pool to signed int

Former-commit-id: 4c4c2f661deb6f70c2c0b3b0bbe1a67ec182a4aa

cpp/src/db/MySQLConnectionPool.h

@@ -94,7 +94,7 @@ protected:
 
 private:
     // Number of connections currently in use
-    unsigned int conns_in_use_;
+    int conns_in_use_;
 
     // Our connection parameters
     std::string db_, user_, password_, server_;