profiler fails on multiprocessing setups

[cboulay]

Reported by @pperanich , enabling the profiler raises errors when running pipelines with multiprocessing enabled.

1. We should include in the documentation that the current profiler is limited in that it only tests how long it takes for a node to process and return a message, and does not provide much information about how the entire system is functioning, so it is inherently limited and only intended to be used in limited debugging scenarios which does not include multiprocessing. This should be done before the next release.
2. We should then make the current profiler and multiprocessing mutually exclusive.
3. We can rip out the current profiling code after the more comprehensive profiler is done.