Introduce the (outside-of-iomgr) pollset API.

This CL introduces the public side of this interface. There will need to be an
iomgr-private API also, but this will be a per-implementation API and so is not
covered here.

I've taken care of wiring the interface through the codebase in the manner that
I expect it will be used.
	Change on 2014/12/17 by ctiller <ctiller@google.com>
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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=82376987