BUG-1196: fixed bug in choice case codec.

BUG-1196: fixed bug in choice case codec.

Implemented lazy generation of choice case codecs by
using DispatchChoiceCodecImpl - delegator for de/serialization
of case instances in
PublicChoiceCodecImpl. This codec is created for each interface
generated from yang choice. It contains information about all
implementations of choice-cases in context and delegate
de/serialization of concrete case instance to correct case
codec based on given InstanceIdentifier.

ChoiceCaseCodecImpl instances are created when concrete case
is encountered.

By using DispatchChoiceCodecImpl we don't need to hold
COMPOSITE_TO_CASE and CLASS_TO_CASE_MAP variables in runtime
generated codec which were
used to serialization and deserialization before.

Added test which test serialization / deserialization.

Change-Id: I52eb78f66e91695758b825b109639d039860f997
Signed-off-by: Martin Vitez <mvitez@cisco.com>