The loops in ExecHashJoinNewBatch(), ExecHashIncreaseNumBatches(), and
ExecHashRemoveNextSkewBucket() are all capable of iterating over many
tuples without ever doing a CHECK_FOR_INTERRUPTS, so that the backend
might fail to respond to SIGINT or SIGTERM for an unreasonably long time.
Fix that.  In the case of ExecHashJoinNewBatch(), it seems useful to put
the added CHECK_FOR_INTERRUPTS into ExecHashJoinGetSavedTuple() rather
than directly in the loop, because that will also ensure that both
principal code paths through ExecHashJoinOuterGetTuple() will do a
CHECK_FOR_INTERRUPTS, which seems like a good idea to avoid surprises.

Back-patch to all supported branches.

Tom Lane and Thomas Munro

Discussion: https://postgr.es/m/6044.1487121720@sss.pgh.pa.us