implemented prepared statement for postgres and sqlite

src/sql/query_intermediates.cpp

@@ -18,6 +18,11 @@ std::unique_ptr<query_result_impl> query_select_finish::fetch()
   return session_.fetch(builder_.compile().sql);
 }
 
+statement query_select_finish::prepare()
+{
+  return session_.prepare(builder_.compile());
+}
+
 query_intermediate::query_intermediate(session &db, query_builder &query)
 : session_(db), builder_(query) {}
 
@@ -103,6 +108,11 @@ std::pair<size_t, std::string> query_execute_finish::execute()
   return session_.execute(builder_.compile().sql);
 }
 
+statement query_execute_finish::prepare()
+{
+  return session_.prepare(builder_.compile());
+}
+
 query_execute_finish query_into_intermediate::values(std::initializer_list<any_type> values)
 {
   return {session_, builder_.values(values)};
@@ -140,10 +150,10 @@ query_execute_where_intermediate query_set_intermediate::where(const basic_condi
   return {session_, builder_.where(cond)};
 }
 
-query_update_intermediate::query_update_intermediate(session &s, std::string table_name)
+query_update_intermediate::query_update_intermediate(session &s, const std::string& table_name)
 : query_start_intermediate(s)
 {
-  builder_.update(std::move(table_name));
+  builder_.update(table_name);
 }
 
 query_set_intermediate query_update_intermediate::set(std::initializer_list<key_value_pair> columns)