Index cleanups

[buffrr]

This PR ensures:

1. get_commitment returns the tip even after rollback if root wasn't specified explicitly
2. Fixes crash for expired spaces. This fix ensures the revoke event is emitted correctly and prevents showing a duplicate revoke event in some cases. The fix is different from PR Fix space revocation bug #122 in the main branch in that it keeps the relevant assertions, and ensures the index is always consistent so there's no need for graceful checks in get_space.