Support for data scanning queries

[ikopylov]

trafficstars

It would be great to add support for data scanning queries to Bob. This will require adding the ability to read data from all nodes and merge the results to avoid duplication, adding a new streaming API, adding some special optimizations for Key-range queries and TimeStamp-range queries.

Most interesting examples:

1. Get all records in TimeStamp range. User specifies minTimeStamp and maxTimeStamp to get the range of data (or the range of keys) stored in Bob
2. Get all records in Key range. The same as above, but is base on the keys
3. Scan all records stored in Bob
4. Count all records stored in Bob