Fold RootCodecContext into BindingCodecContext

Fold RootCodecContext into BindingCodecContext

RootCodecContext exists only for two purposes:
- return for an empty YangInstanceIdentifier lookup
- ease of InstanceIdentifier lookup

In order to fulfill these two roles, it ends up twisting the
BindingDataObjectCodecTreeNode contract in rather nonsensical ways,
preventing a number of further evolutions of the API.

Fold its functionality into BindingCodecContext, eliminating it from the
class hierarchy.

Change-Id: I44a49b78ed9ae418c5c85e48e9441fa1ec1e9862
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>