Clustering : Add support to allow reads from followers

The Clustered Data Store ensures that all reads are consistent by routing reads to the Shard Leader. This obviously has an adverse impact on application performance. We need to provide a mechanism in the API or via configuration so that application developers and/or administrators can choose to allow reads from a follower.

It goes without saying that applications that choose to use this mechanism will have to deal with the situation where a ReadTransaction following a WriteTransaction may not see the data written by the WriteTransaction.

Also the support to read from a follower cannot be implemented when transactions occur on a TransactionChain.