Merge seastar upstream

* seastar 93c3b12...7790e68 (7):
  > core/reactor: Introduce reactor-*/dervie-busy_ns metric
  > Collectd: Hold a reference to the metrics implementation in registration
  > future: Improve comments
  > fstream: actually use dynamically adjusted buffer
  > debug: add latency detector script
  > reactor: add static probes for latency detector
  > semaphore: Fix with_semaphore() in case wait() throws