Fixes: SCYLLADB-1693
In case we abort a decommission operation, the snapshot/backup
mechanism need to remain open.
This change moves it to after raft_decommission.
In the case of a cluster snapshot, our nodes ownership
or not of tables will be serialized by raft anyway, so
should remain consistent. In that case we at worst coordinate
from a node in "leave" status
In the case of a local snapshot, ownership matters less,
only sstables on disk, which should not change.
In the case of backup, this operates on a snapshot, state of which
is not affected.
Adds an injection point for testing.
v2:
- Added injection point to ensure test can abort decommission
Closesscylladb/scylladb#29667
2cc1a2c406