The leader_host API handler was eventually using the `req` unique_ptr
after it has been already destroyed (passed down to the future lambda
by reference). This was causing an occassional crash in some tests.
Reworked the leader_host handler to use the req only outside of the
future lambda.
Also updated the code to handle the possibility that the non-default
leader group (other than Group 0) might reside on a different shard
than the shard 0 - using the same concept of calling on all shards via
`invoke_on_all()` as done for the other requests.
Fixesscylladb/scylladb#19714Closesscylladb/scylladb#19715
144794a952