Simplify the function by converting it to a coroutine.
Note that while the final co_return co_await looks like a loop (and
therefore an await would introduce an O(n) allocation), it really isn't -
we retry at most once.
This is done to ease code reuse in the following commit.
It'd also help should we ever want properly mount functions
class to schema object instead of static storage.
forward_service is nondescriptive and misnamed, as it does more than
forward requests. It's a classic map/reduce algorithm (and in fact one
of its parameters is "reducer"), so name it accordingly.
The name "forward" leaked into the wire protocol for the messaging
service RPC isolation cookie, so it's kept there. It's also maintained
in the name of the logger (for "nodetool setlogginglevel") for
compatibility with tests.
Closesscylladb/scylladb#19444
