[feature] add path partitioning to namespaces

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

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