As they are wasteful in many cases, it is better
to move the tablet_map if possible, or clone
it gently in an async fiber.
Add clone() and clone_gently() methods to
allow explicit copies.
* minor optimization, no backport needed
Closesscylladb/scylladb#24978
* github.com:scylladb/scylladb:
tablets: prevent accidental copy of tablets_map
locator: tablets: get rid of synchronous mutate_tablet_map