Modeled after get_live_members_synchronized,
get_unreachable_members_synchronized calls
replicate_live_endpoints_on_change to synchronize
the state of unreachable_members on all shards.
Fixes#12261Fixes#15088
Also, add rest_api unit test for those apis
Closes#15093
* github.com:scylladb/scylladb:
test: rest_api: add test_gossiper
gossiper: add get_unreachable_members_synchronized
(cherry picked from commit 57deeb5d39)
Backport note: `gossiper::lock_endpoint_update_semaphore` helper
function was missing, replaced with
`get_units(g._endpoint_update_semaphore, 1)`
ca8723a6fd