SERVER-15675 Remove OperationContext from UpdateRequest/DeleteRequest

The write execution machinery should have a handle to the operation
context, not the write request structs.
SERVER-15675 PlanStage::invalidate() needs OperationContext

PlanStage::invalidate() is always called by a different thread than
the stage's owning thread.  The method should use the active
OperationContext (the caller's) rather than the stage's
OperationContext.
SERVER-15675 Stages should clear their OperationContext in saveState()

While state is saved, threads pass in their own OperationContext to
invalidate().  When state is restored, the stage will resume with the
OperationContext passed in to restoreState().