ScanSink is not threadsafe #2679
Description
There if of course no direct guarantee that operators should be thread safe in the Rx realm, and it is a matter of implementation, but at the same time the precedent of having locks inside Sinks has been set a long time ago - just look at the date on SynchronizedOnType, and it's of course used in a lot of places.
ScanSink maybe probably definitely should be internally synchronized since it owns a mutable state...