_iter_parse_timed_blocks speedup

Rewrite _iter_parse_timed_blocks so it is almost as fast
as _parse_timed_blocks (only a few percent slower, while
previous version was 2-3 times slower).
more use _iter_parse_timed_blocks

Use _iter_parse_timed_blocks for parsing diskstats, proc_stat
and meminfo. Should reduce memory footprint for new versions
of bootchart data.

It is save to use _iter_parse_timed_blocks, because it is
almost as fast as _parse_timed_blocks now.