fair_queue: Improve latency spikes on preemption
fair_queue: Improve latency spikes on preemption

Reduce the decay to 5ms and compensate too frequent normalization by
expanding the accumulator type to be double.

Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@scylladb.com>

scylladb/seastar,scylladb/seastar,avikivity/seastar,syuu1228/seastar,avikivity/seastar,avikivity/seastar,scylladb/seastar,syuu1228/seastar,syuu1228/seastar