Increase session saving chunk size for much better performance.

Increasing the StoreSession chunk size from 1Msamples to 10Msamples
leads to massively faster session saving due to reduced overhead.

In some cases a 2x (or more) speed-up has been observed. A further increase
to, say, 100Msamples shows no further noticeable improvements.

This also has the additional benefit of generating fewer entries/files
within the sigrok session (*.sr) output/ZIP files, which can potentially
be beneficial for very long captures.