Tide SST

[aliciayuting]

SST support for upper level application, Cascade scheduler TIDE, to periodically update each node about the most recent loading situation for all nodes.

1. Added TIDE column in SST from multicast group.
2. Created two buffers at group, active buffer and a shadow buffer. So that the upper level application will always update the active buffer and read from the shadow buffer.
3. Defined predicates at multicast_group, to periodically update the active buffer, then flip the load_info_active flag.

[songweijia]

Need some work

• Change the shared state definition: it should be a structure.
• A little rework on the swap code.
• Let's put it in the 'load_monitor' branch first.