BUG-1431: make sure (Yang)InstanceIdentifier is Serializable

This works around the fact that an Iterable cannot be serialized by
forcing instantiation of the legacy path. The read side then just does
the opposite.

For InstanceIdentifier the situation is similar, except we perform the
writeout/read-in ourselves.

Bumps the serialization format for InstanceIdentifier and
YangInstanceIdentifier to 2 and 3 respectively.

Change-Id: I668e2aeebde20b99cfe3b41594b3925697295d05
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "BUG-1431: make sure (Yang)InstanceIdentifier is Serializable"