ENH: add chunksize argument to read_sql (GH2908)

Closes #2908 

Should probably add this also to `read_sql_table`. And try to reduce some repetitions (`_wrap_result` can maybe be a top-level (in the file) helper function).
the reason is that it didn't use `self`, so I made it explicitely static. Problem is that the version *is* different for the two classes, so that's why I put it in the class instead of just a general helper function like `_wrap_result`