Add fences

[anlun]

• To support acquire-fences one needs to add another per-thread front, ψ_acq​​ .
  The front would store a union of synchronization fronts from messages read by the thread,
  including relaxed reads, which don't use corresponding synchronization fronts in the current version.
  When an acquire-fence is executed, the thread's viewfront will be updated to ψ_acq.
• For release fences we would need to modify a per-thread write front state component.
  Each thread would explicitly store a front for every observed location instead of storing a pointer
  to the latest release write, which contains the same front, to the location.
  An execution of a release-fence would update these fronts to match the thread's own viewfront.
• Similarly, an SC-fence, when executed, updates the corresponding thread's viewfront by the
  global SC front.

Also, one would have to adapt the rules for postponed operations accordingly, ie, forbid out-of-order resolving of an acquire-fence and a read from a postponed operation buffer.