Tests entrypoint was in a testing/ subfolder in the root of the
repository. This was made mainly because we had some common files for
tests and we didn't want to ship them (files in testing/test_soledad,
which is itself a python package. This sometimes causes errors when
loading tests (it seems setuptools is confused with having one python
package in a subdirectory of another).

This commit moves the tests entrypoint to the root of the repository.

Closes: #8952

Hardcoded and repeated user uuids can lead to accidental concurrent
operations between test cases, breaking isolation and creating random
failures. This commit improves it a bit.

Adds two new columns for sync status and retries. Also some initial
rough logic for upload retry limiting.

-- Resolves: #8823
-- Related: #8822

This value was hardcoded on client, but it's assumed to be default by
the server and there is no need for it to be hardcoded.

-- Resolves: #8882

A reported bug on namespace feature was that we couldn't delete a
namespaced blob after a cold start, since the client wasn't able to
check which namespace it belongs.
This commits completes the tracking of namespace over client site code,
making it possible to query and store namespce information on disk,
through sqlcipher.

-- Resolves: #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

IncomingBox spec has a flags feature for the processing flow of
messages. This commit adds it using a .flags file.

-- Resolves: #8869

Listing by date is useful for listing newest/oldest documents on blobs
storage and should be used for listing new IncomingMessages as described
on specification.

-- Resolves: #8879

Adds an extra parameter called "namespace" on the backend interface and
on FileSystemBlobsBackend. This parameter overrides default id
partitioning and uses a separate folder for a custom namespace.

-- Resolves: #8889