Client chosen resource identifiers need to be unique (duplicate checking and idempotency)

[bushjames]

Item ET1 from vNext beta assessment report:

Resource identifiers must be unique within a scheme and enforced by the hub.

Alpha comments: Duplicate checking on identifiers e.g. transferID is a requirement of the specification. Checks must be made by the hub on all incoming client chosen resource identifiers to ensure no duplicates are introduced to schemes. This has to be since the beginning of scheme time.

Beta comments: Some code comments added but no concrete implementation since alpha

Concrete implementation needed for all resource types.