execution: bugfix - backtrack seq-cst fences properly

execution: bugfix - backtrack seq-cst fences properly

The commented-out ATOMIC_FENCE should not be commented out; the note
that "fences don't directly cause backtracking" is false for seq-cst
fences. We intend to backtrack for all seq-cst fences, so we need to
perform the appropriate 'get conflict' search for all fences.