Updated Newtonsoft.Json to v6.0.2 and Microsoft.Bcl.Immutable to 1.1.20-beta
Bulk operations: refactor FdbBulkOperations into Fdb.Bulk static class (work in progress)

* Fdb.Bulk.WriteAsync(...) to insert large quantities of k/v into a db
* Fdb.Bulk.ReadBatchedAsync(...) with multiple overrides to be able to read/process large runs of data
Ranked Set Layer: initial port of the python layer (work in progress)

- warning: there is a known bug due to snapshot reads
A few cosmetic fixes