Exposes event#finish as #close.

- ensures that it's a runtime error if an event if used after it's finished

- updates all calls where the completion_queue is used to ensure the event's
retrieved are explicitly finished
	Change on 2014/12/18 by temiola <temiola@google.com>
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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=82445748