BUG/MEDIUM: namespace: close open namespaces during soft shutdown

BUG/MEDIUM: namespace: close open namespaces during soft shutdown

When doing a soft shutdown, we won't be making new connections anymore so
there's no point in keeping the namespace file descriptors open anymore.

Keeping these open effectively makes it impossible to properly clean up
namespaces which are no longer used in the new configuration until all
previously opened connections are closed in the old worker process.

This change introduces a cleanup function that is called during soft shutdown
that closes all namespace file descriptors by iterating over the namespace
ebtree.