Added chunksize argument to to_sql

Large dataframes may fail to write to a database in one go due to packet size errors .  This add a `chunk_size` parameter which will write the dataframe to the database in batches.  See https://github.com/pydata/pandas/issues/7347 .

Notice that there is quite a bit of code duplication between the SQLAlchemy and legacy APIs. This could be factored out if desired at the cost of a few private methods.

Closes #7347
why is this line added? I don't see keys used further in the function (maybe copied by mistake from the sqlalchemy version?)