Fix latency accuracy and dumping latencies to file

Fix latency accuracy and dumping latencies to file

- Also changed lat_info.tx_packet_index to uint64_t,
  so also did it for task_lat.prev_tx_packet_index and n_loss.
- Adjusted format strings accordingly, and fixed some other formats.
- Adjusted overflow increment to 2^32 (i.e. UINT32_MAX + 1).
- Replaced hard-coded 64 with ACCURACY_BUFFER_SIZE (still hard-coded
  in handle_gen.c).

Change-Id: Ia59f36e17c0797a2a958dbe3b2ac420263473524
Signed-off-by: Xavier Simonart <xavier.simonart@intel.com>
Signed-off-by: Patrice Buriez <patrice.buriez@intel.com>