run_set
run_set.negate()
[run_set] improve run merging
[run_set] add another unit test
[subtracting_span_iterator] recode using run_set
[features] test for thin_dump with small metadata
[metadata_dumper] Fix bug where ranges were appearing in all devices for small metadata.

Author: Teng-Feng Yang
[run_set] use lower_bound to skip preceeding entries more quickly.
remove old FIXME
stop using O_DIRECT until the cach does async io
create_xml_data: fix nr_data_blocks
create_xml_data: fix device and range_mapping
thin_metadata_size,create_xml_data: fix units
thin_metadata_size: drop signe_mapping to =
thin_metadata_size: check for block size > 0
create_xml_data: support --size-variation and check for block size > 0
create_xml_data: remove space in dev_id ouput
create_xml_data: adjust filed order to thin_dumo format; fix nr_data_blocks
create_xml_data: adjust to thinp_xml output
create_xml_data: fiddling