Bug 2265: Address comments from 12448

Bug 2265: Address comments from 12448

Address comments from https://git.opendaylight.org/gerrit/#/c/12448/:

- CanCommitTransactionReply: "you could have used a CONSTANT here
YES_SERIALIZED, NO_SERIALIZED. Or even completely eliminating the
serializedMessage field"

- MergeData: "We could have use SERIALIZABLE_CLASS.isInstance(message)
here and in lot of other places where we are checking the message
type with equals now."

Change-Id: I8248fb739b45a9ad4795b355f3b4ef9291e99791
Signed-off-by: tpantelis <tpanteli@brocade.com>