unit/manifest streaming to minimize memory footprint.
update unit tests.
rename query to conduit.
add len() support back into the unit iterator; docstrings.
make sure manifest is cleaned up; add index filter to the units iterator.
use UnitRef appraoch instead.
Merge branch 'master' into jortel-960159

Conflicts:
	nodes/common/pulp_node/manifest.py
move nectar in the import ordering.
simplified download module and updated docstrings.
fix unit tests.
consolidate the manifest back into a single object; better semantics.
document the exceptions.
docstrings
docstrings
Merge branch 'master' into jortel-960159
copy buffer tuning.
refactor compression.
decompress files when compression is detected.
avoid cross-device linking.
Better docstring.