OClCollMeshEngn: Add bridged delay in finalize()

See the diff of the todo file within this commit for more details.

In short, we do this to prevent the possibility of an in-flight async
contin accessing metadata that we've already destroyed after finalize()
has been called.

todo

@@ -38,18 +38,7 @@
 
 	PcloudStimProducer::stop=>start() sequence:
 
-		OpenClCollatingAndMeshingEngine::finalize():
-I'm also worried, though less so, about the OClCollMeshEngn: it's
-a lot less likely to have an in-flight op run past the point where
-the OClCollMeshEngn object has expired.
-* But there's still a chance that a long-running OCl kernel could
-  cause an in-flight async contin to resume executing after its
-  OclCollMeshEngn has expired.
-* We should do a bridged async wait for the std::max() of all
-  timeouts used by OClCollMeshEngn to pass before leaving
-  PcloudStimProducer::stop.
-
-	Attaching and detaching StimBuffs from StimProducers:
+		Attaching and detaching StimBuffs from StimProducers:
 We've written code recently to attach and detact stimBuffs from a
 stimProducer. The code is quite nice, but there's this hanging
 omen over the fact that we put no thought into ensuring that