Contribution - Optimize usage of GridCacheConcurrentMap (IGNITE-2948)

[yzhdanov]

Guys,

I have finally reviewed and accepted contribution made by Ilya Lantukh.
That was pretty useful change - it removes data duplication (after years of
having one!), i.e. earlier we had to store cache entry by reference in 2
maps, now only in 1. It brings up to 20% for memory efficiency (this
depends on size of keys and values).

Thanks to Ilya and other reviewers - Sam and Alex Goncharuk!

Here is link to ticket - https://issues.apache.org/jira/browse/IGNITE-2948

--Yakov