Backport InstallSnapshot message serialization changes

Backport InstallSnapshot message serialization changes

Backported https://git.opendaylight.org/gerrit/#/c/33768/ and
https://git.opendaylight.org/gerrit/#/c/33767/ from master to
eliminate the protobuff serialization to make it easier to change
the serialization. A subsequent patch will add a new field to
InstallSnapshot. Backwards compatibility with versions prior to
Be SR3 is maintained, ie protobuff serialization will still be used.

Change-Id: I465daba0b83e35bfe0e0d5c345a497dd7f9425d4
Signed-off-by: Tom Pantelis <tpanteli@brocade.com>