Recently there had been added add_tablet_replica and del_tablet_replica API calls that copy big portion of the existing move_tablet API call's logic. This PR generalizes the common parts
Closesscylladb/scylladb#18272
* github.com:scylladb/scylladb:
tablets: Generalize transition mutations preparation
tablets: Generalize tablet-already-in-transition check
tablets: Generalize raft communications for tablet transition API calls
tablets: Drop src vs dst equality check from move_tablet()