Bug 2363, Bug 2205. Beta version of LeafRefContext tree computation

Bug 2363, Bug 2205. Beta version of LeafRefContext tree computation

beta version of DataTree leafrefs values validation

Currently works (Bug 2363):

- LeafRefContext tree build with all relations:
          - isReferencing/ hasReferencingChild
          - isReferenced/ hasReferencedChild
          - leafref target node -> leafref source nodes
- validation of LeafRef Path and Predicates syntax according to RFC6020
  specification
- parsing of all LeafRef Path components (i.e.: LeafRef absolute/relative
  target path, path Predicates, key path expressions) into LeafRefPath
  instance
- pack of utils methods over LeafRefContext tree
- a couple of simple tests

Currently works (Bug 2205):

- validation of LeafRefs values and LeafRefs targets for WRITE, MERGE and
  DELETE operations
- leafref path predicates evaluation (also multiple)

Currently known issues (Bug 2363):

- improve leafref identification, also for cases when type of the leaf is
  typedef -> typedef -> etc.. -> leafref
- handle also choices without case
- questionable issue is, whether it is necessary to merge referencing and
  referencedBy subtrees under the root context (currently are not)
- in equals method check also for predicates equality
- test the solution

Currently known issues (Bug 2205):
- handle also UnkeyedLists
- perfrom further testing
- increase unit tests coverage
- add tests for special cases.

Change-Id: Id873fb446fe40855993fe00e6a56f82d3daee491
Signed-off-by: Peter Kajsa <pkajsa@cisco.com>