executemany in pd.to_sql?
Currently, the generic SQLTable uses `execute` rather than `executemany`. I'm fairly green on these issues, but my understanding is that it's materially slower, depending on the driver used.
https://github.com/pydata/pandas/blob/master/pandas/io/sql.py#L745

The SQLite version _does_ use `executemany`: https://github.com/pydata/pandas/blob/master/pandas/io/sql.py#L1441

Does anyone know why this uses `execute`? Or whether my assumptions on the speed difference are incorrect?

CC @zzzeek
