Ability to specify the timestamp generator timeout extra delay

This timeout extra delay is used to compute the consistency delay. After the consistency delay, a transaction is flag as consistent i.e. we have seen all the transactions up the transaction timestamp. The timeout extra delay is a buffer to process the timestamp generator timeout response.
Merge pull request #217 from wajam/manu-consistency-timeout-delay

Ability to specify the timestamp generator timeout extra delay
code review fixes, fixed comments too
Merge branch 'master' into jn-cluster-split-support
added subscription on new shard (which does nothing for now)