Create a simplified status for offetset

[Zarquan]

Create a new simpler status flag for offerset {OFFERED|ACCEPTED|REJECTED}.
The status of an offerset reflects the status of the sessions it contains.

Offerset status starts as OFFERED.
If at least one of the sessions are OFFERED, then the offerset status changes to OFFERED,
If one of the sessions is ACCEPTED, then the offerset status changes to ACCEPTED,
If all of the sessions are REJECTED, then the offerset status changes to REJECTED,

If the offerset status is OFFERED, then the client may update the status to REJECTED.
Doing so will update all of the sessions to REJECTED.