Tablet + Raft changes

[mflendrich]

Issue originally authored by tnozicka as #1676

When raft and tablets land in ScylalDB we'll have to revise existing procedures and change mane of them. E.g. adding multiple nodes vs StatefulSets.

• Use Parallel Pod Management Policy in StatefulSet
• Remove waiting until previous rack (sts) is fully rolled out before reconciling next one
• Change seed selection algorithm to account for case when multiple nodes are added at once

[mflendrich]

@zimnx I need your help triaging this (it is my understanding from previous conversations that we're more or less done here).

Please either close this as completed / no-op, or provide a (very) high level list of stuff that needs to happen (and feel free to unassign yourself then). Thanks

[zimnx]

Added three changes we need to make in order to take benefit from Raft topology coordinator.
With regards to tablets there's nothing to be done on Operator side, just enjoy the ride if users are using them.