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.

src/session_driver.c

@@ -31,7 +31,7 @@
 
 /* size of payloads sent across the session bus */
 /** @cond PRIVATE */
-#define CHUNKSIZE (512 * 1024)
+#define CHUNKSIZE (4 * 1024 * 1024)
 /** @endcond */
 
 SR_PRIV struct sr_dev_driver session_driver_info;