[Segment Replication] Swap role of primary and replica shard

[dreamer-89]

Coming from disucssion in #6210 which introduced primary shard based balancing, may need extra relocations (example 1) to attain complete balance and in some situations relocation is not at all possible (example 2). This can be fixed with minimal (single) operation by exchanging primary and replica shard roles. This needs to be done under primary shard permit and ensure this change is committed to cluster state. This is already tried for searchable snapshot, can be attempted for hot shards as well.

Problem 1 (More relocations)

Node 1	Node 2
P1	R3
P2	R4

Node 1	Node 2
P1	R3
	R4
	P2

Node 1	Node 2
P1	R3
R4	P2

Problem 2 (No relocation possible)

Node 1	Node 2
P1	R2
P2	R1