Bug #647: Add missing CkMigrateMessage constructors for *MeshStreamer classes
Without these constructors being defined, attempts at restoring these types
from a checkpoint would have crashed at runtime. This suggests that the
boilerplate-reduction change in
7af4a4cae8fa092e8b8f054c5d0976c14b0ca6df should
be limited to chare arrays, where the migration constructor is optional, and
should always be required for explicitly marked [migratable]
chare/group/nodegroup types.
Change-Id: Ibab14fa33b0cf80c7c5f5fd273868c8bd81503da