API updates for #41

This version retains all the old messages, although they are *deprecated*. We can remove the deprecated messages when everything using them is updated.

The API is backwards compatible, so it *should* be safe to merge. There are new fields and constants, so the MD5 checksum will change, forcing recompilation.

Changes:

- add new `SchedulerRequests` message type 
- deprecate `AllocateResources` and `SchedulerFeedback` 
- add new `RESERVED` status to `Request` message 
- add `hold_time` duration to `Request` message