Add "Read-Concern" optional flag to queries and transactions

[tock-ibm]

Read-Concern determines from which server in the cluster to read data:

• Leader: only from the leader, if hits another server redirect/reject
• Follower: a server that is not a leader, but knows who the leader is, so in high probability it is in the quorum (majority group)
• LeaderOrFollower: either a leader or a follower, as defined above.
• Any: can be any server, even a server that does not know who the leader is and might be in a minority group, with stale data.

The default behavior when the flag is missing is "Any".

Note that TXs are directed to the leader when submitted, as only the leader can propose TXs for ordering. This flag relates to reads prior to commit.

This issue is to add the flag to the REST API, other issues will implement server behavior.