session_driver.c: Increase chunk size for slightly better performance.

Increasing the chunk size from 512ksamples to 4Msamples leads to
slightly faster session file loading from sigrok session (*.sr) files
in frontends, while not looking too "chunky".

The performance increase is not really massive, but still noticeable.
A further increase to, say, 10 or 100Msamples shows no further
noticeable improvements.