PlatformTransactionManager implementation for TokuMX, a MongoDB Engine

[TokuMX][tokumx] is relatively new MongoDB backend supporting Multi­ Statement Transactions (ACID and MVCC transaction support). The pull request contains an implementation of {@link PlatformTransactionManager} for TokuMX which I would like to contribute to spring-data-mongo.

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[tokumx]: http://www.tokutek.com/products/tokumx-for-mongodb/