Add a simple node state model with:
`joining`, `normal`, `leaving`, and `left` states
to help managing nodes during replace
with the the same ip address.
Later on, this could also help prevent nodes
that were decommissioned, removed, or replaced
from rejoining the cluster.
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
c17df1759e