service: misc_services: fix lowres_clock::duration assumption
service: misc_services: fix lowres_clock::duration assumption

recalculate_hitrates() is defined to return future<lowres_clock::duration>
but actually returns future<std::chrono::milliseconds>. This fails
if the two types are not the same.

Fix by returning the declared type.

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla