Refactored Future to use shared_ptr.
Refactored Future to use shared_ptr.

Replaced manual reference counting with shared_ptr in Future. This
allows us to kill some nasty copy and deallocation functions in Future
(e.g. cleanup, copy).

From: Jie Yu <0acc13d62cce94f85218ac2e2aa40486e36f9a9f@gmail.com>
Review: https://reviews.apache.org/r/13602

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