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
