added batched authorizeSpend calls.

When the number of accounts grows, the SlaveBanker ends up initiating a large amount of REST requests to authorize further spend and report the spend of each account individually. The goal of this branch is to batch these requests under a single call thus reducing the overall communication overhead.
https://github.com/datacratic/rtbkit/pull/103/files#diff-08d5d3aef789158169fb00a70eeb7c0aR191