INT-3143: Add support to allow configuring MongoTemplate with MessageStore (Review Only)

This is a Preview PR for the work in progress of the ``ConfigurableMongoDbMessageStore``. The current implementation passes all the test cases for the existing stores. Following are the objectives of this review.

* Check if the approach for the new Store is ok.
* Suggest/Discuss any additional Junits those would be needed to make the tests more comprehensive
* Other miscellaneous coding practices.

The documentation of this change is not yet done and should be taken care once the code review is done. The changes will be pushed in the same PR as a separate commit.

The Jira ticket for this PR is  https://jira.springsource.org/browse/INT-3143