- Deprecate slapos.networkcachehelper
- Common internal method to query network cache, to reduce code duplication
  and fix downloading though SSL.
- Do not upload data that is already in SHACACHE. This reverts commit
  7bb5e112 partially, since it's not possible
  to hash on the fly. 'tempfile' module is reimported for non-seekable streams.
- New way to instanciate NetworkcacheClient, again to reduce code duplication,
  but also to use an instance for both upload & download.
  Old way is still there for compatibility until it is unused.