borg2: give complete levels / namespaces configuration to borgstore

[ThomasWaldmann]

This is blocked and moved to borgstore 0.1.0 milestone due to borgbackup not yet doing this (as of b11), but relies on the levels == [0] default:

borgbackup/borgstore#67

So, for borg b12:

• fix this issue in borg
• release borgstore 0.1.0
• require borgstore ~0.1.0 in borg

When borgstore starts using this to pre-create namespace directories:

• repository creation will get slower then, because it will pre-create a lot of nesting directories
• archive creation will get faster on some backends due to them not having to do backend.mkdir calls anymore.

[ThomasWaldmann]

Issue is fixed in borg 2.0.0b12, but keeping it open for the rest of the TODO.