CONCPP-70 statement::addBatch test sporadically failed

The reason was, that filling of vector with executeBatch results was
done using undefined behavior - it was done using [] operator member
access for not existing members. The values were inserted correctly, but
the size of vector happened to not always be reported correctly.
Added similar test to preparedstatement.
Made clearBatch not to reset current set of parameters - that looks
like to be that jdbc specs expect.
Fixed other such cases of wrong vector use.
Made some to/from text conversion to use C locale.

install_test/example.cpp

@@ -20,13 +20,16 @@
 #include <iostream>
 #include <memory>
 #include <regex>
+#include <string>
+#include <sstream>
+#include <cmath>
 
 #include "mariadb/conncpp.hpp"
 
 int main(int argc, char** argv)
 {
-    // Instantiate Driver
-    sql::Driver* driver = sql::mariadb::get_driver_instance();
+  // Instantiate Driver
+  sql::Driver* driver = sql::mariadb::get_driver_instance();
 
   // Configure Connection
   sql::SQLString url("jdbc:mariadb://localhost/test");