Ensure that every string key and value of the json response from the network cache server is a str object

Please, @kazuhiko and @kirr, review this.

See merge request !1

Ensure that every string key and value of the json response from the network cache server is a str object.

This method was added mostly because the NetworkcacheClient gets a json response from the server and all textual keys and values after json.loads are unicode objects, which may result in a error later when we try to join the url and the parameters of the package to use the networkcache.

We can't depend on pyOpenSSL because SlapOS is not ready for that.

To simplify things, DirectoryNotFound & UploadError are merged in
NetworkcacheException.

This also fixes a bug returning wrong metadata when several entries exists
in shadirs.

- 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.

For consistency with other options, and because there's already some code in
the wild using dashes instead of underscores.

If people don't write stupid code like 'if result is True:',
this small API change should not break anything.

Add new index() method to add an entry to shadir when the data is known to be
already in shacache.

When uploading big file, this avoids trashing the kernel cache once more.