Use multi-row inserts for massive speedups on to_sql

As suggested here - pandas-dev#8953

Author: nmichaud

pandas/io/sql.py

@@ -700,7 +700,7 @@ def insert_data(self):
 
     def _execute_insert(self, conn, keys, data_iter):
         data = [dict((k, v) for k, v in zip(keys, row)) for row in data_iter]
-        conn.execute(self.insert_statement(), data)
+        conn.execute(self.insert_statement().values(data))
 
     def insert(self, chunksize=None):
         keys, data_list = self.insert_data()