input/csv: Send larger datafeed chunks, to speedup import

input/csv: Send larger datafeed chunks, to speedup import

The previous implementation sent one sigrok session datafeed packet per
processed CSV line. This is rather inefficient for the CSV input module,
and triggers a dramatic performance loss in the srzip output format.

Communicate up to 128K samples within one datafeed packet. This fixes
bug #695.

Factor out repeated calculation of the unit size which is derived from
the channel count. Fix a minor memory leak in an error path while we are
here. (Other memory leaks in rare error paths remain with this commit.)

Suggested-By: Elias Oenal <redacted>