Bug 500: Updated data APIs with new concepts.

Bug 500: Updated data APIs with new concepts.

  - Logical Datastore Type idenfier
  - Asynchronous Data Broker contracts - some data broker contracts
    were synchronous which could misslead consumers of APIs

  - New Transaction subtypes with support for Asynchronous reads
    - read-only transaction
    - write-only transaction
    - read-write transaction
  - Scoped Data Change Publisher - client code is able
    to specify scope of changes and logical datastore
    type on which it is listening.

Change-Id: I2367e7df633dd07b08550014bed7a35c3e88a7ba
Signed-off-by: Tony Tkacik <ttkacik@cisco.com>
Signed-off-by: Robert Varga <rovarga@cisco.com>