OPTIM: sink: reduce contention on sink_announce_dropped()

OPTIM: sink: reduce contention on sink_announce_dropped()

perf top shows that sink_announce_dropped() consumes most of the CPU
on a 128-thread x86 system. Digging further reveals that the atomic
fetch_or() on the dropped field used to detect the presence of another
thread is entirely responsible for this. Indeed, the compiler implements
it using a CAS that loops without relaxing and makes all threads wait
until they can synchronize on this one, only to discover later that
another thread is there and they need to give up.

Let's just replace this with a hand-crafted CAS loop that will detect
*before* attempting the CAS if another thread is there. Doing so
achieves the same goal without forcing threads to agree. With this
simple change, the sustained request rate on h1 with all traces on
bumped from 110k/s to 244k/s!

This should be backported to stable releases where it's often needed
to help debugging.