A motion hazard is a deadlock between motions, a classic motion hazard
in a join executor is formed by its inner and outer motions, it can be
prevented by prefetching the inner plan, refer to motion_sanity_check()
for details.

A similar motion hazard can be formed by the outer motion and the join
qual motion.  A join executor fetches a outer tuple, filters it with the
join qual, then repeat the process on all the outer tuples.  When there
are motions in both outer plan and the join qual then below state is
possible:

0. processes A and B belong to the join slice, process C belongs to the
   outer slice, process D belongs to the JoinQual slice;
1. A has read the first outer tuple and is fetching tuples from D;
2. D is waiting for ACK from B;
3. B is fetching the first outer tuple from C;
4. C is waiting for ACK from A;

So a deadlock is formed A->D->B->C->A.  We can prevent it also by
prefetching the join qual.
Reviewed-by: NJesse Zhang <jzhang@pivotal.io>
Reviewed-by: NGang Xiong <gxiong@pivotal.io>
Reviewed-by: NZhenghua Lyu <zlv@pivotal.io>