Sensible equals and hashCode for FragmentEntity (CPS-1664 #1)

Include Anchor and Xpath in equals and hashCode methods for
FragmentEntity. (This also requires adding equals and hashCode for
AnchorEntity and DataspaceEntity.) The combination of dataspace,
anchor, and xpath uniquely identify a fragment/datanode.
This allows FragmentEntity objects returned from query across anchors
to be stored in Set collections.
Performance was observed to be unaffected by the change.

Issue-ID: CPS-1664
Signed-off-by: danielhanrahan <daniel.hanrahan@est.tech>
Change-Id: I2c7e3957e392af36f5230d08c9bbd550f44e7444