Add statistics on the client to measure and report latency of operations.

The current llite.*.stats file only reports the operation counts, not the time taken for each one. It should be simple to add stats to measure min/max/sum/sumsq for these metrics, including new read and write operations for the latency, as currently it reports read_bytes and write_bytes, which should also be kept. It might make sense to report sync writes separately as write_sync (with file->f_flags & (O_DIRECT | O_SYNC) set) since the latency profile will be quite different compared to cached writes.