Also send Refresh and Flush actions to relocation targets

Currently we send relocation & flush actions based on all assigned ShardRoutings. During the final stage of relocation, we may miss to refresh/flush a shard if the coordinating node has not yet processed the cluster state update indicating that a relocation is completed *and* the relocation target node has already processed it (i.e., started the shard and has accepted new indexing requests).

Closes #6545