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

This column will keep track of namespace locally.

-- Related: #8882

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

Adds a IncomingBox implementation that can be used by the incoming loop
to interact with the server. Includes end to end test from message
creation on Incoming API to callback consumer.

-- Related: #8914