This commit is complementary to the previous one. It adds a test for the
reported bug (listing default namespace was listing others) and fixes
it.

-- Related: #8882

All blobs were being stored in a single folder when using namespaces,
this commits adds path partitioning as discussed on #8882, which should
help with a large number of files (each folder will hold a smaller
subset, allowing the use of better filesystem walk strategies).
Also, the default empty namespace is now called 'default' to prevent it
from listing other namespaces contents. So everything will always use
namespaces, with the option to use it explicitly or just fall to the
default one.

-- Related: #8882

IIncomingBoxBackend holds backend specific methods used for IncomingBox
implementation.

- Resolves: #8888

Clients will query for blobs flagged PENDING so they can start
processing. This commit adds flagging, but still in a hacky way as the
backend requires the flags as a json string inside a request. A refactor
to separate request handling from the backend itself will solve it in
the near future.

-- Related: #8874

Improves error handling, add missing header to __init__.py and remove
mkdirs from flags methods. This is a commit from code review on !117.

Improve test naming, creates a "count" method and filter flags files
properly.

filter_flag parameter can now be used to filter listing blobs by a
specific flags. Eg: I can ask for blobs on incoming namespace flagged as pending.

-- Resolves: #8913

By using `only_count=True` parameter, instead of returning a full list,
server just count the amount of blobs.

-- Resolves: #8871