Add String to known-immutable classes

All instances of String are guaranteed to be immutable, not sure why
that was not captured anywhere.

Change-Id: I9bb2bdd91a0f54e6a8083c047cfd99c0cd029b4b
Signed-off-by: Robert Varga <rovarga@cisco.com>
Implement DatastoreIdentifierBuilder

This is straightforward enough, so make it work with BA clients.

Change-Id: I1a5340f1ac72148e11cb54554acbb49540251f88
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge changes I1a5340f1,I9bb2bdd9

* changes:
  Implement DatastoreIdentifierBuilder
  Add String to known-immutable classes