Today IndexRoutingTable$Builder#build constructs an all-new IndexShardRoutingTable for each shard on every change. High-traffic clusters may have large numbers of shards per index, and today's behaviour yields many duplicate IndexShardRoutingTable instances which together may consume a substantial amount of heap on the elected master.

Moreover IndexRoutingTable is a SimpleDiffable so we must transmit the whole table over the wire on any change, which prevents sharing on the receiving node and therefore yields many duplicate IndexShardRoutingTable instances on the other nodes in the cluster.

We should:

• Re-use existing IndexShardRoutingTable instances where possible when adjusting the routing table on the elected master
• Refine the diffability of the IndexRoutingTable so that it avoids sending unchanged IndexShardRoutingTable instances over the wire.

Relates #77466