Partitionwise join proposes add a concept of child join relations,
which will have the same relationship with join relations as "other
member" relations do with base relations.  These relations will need
some but not all of the handling that we currently have for join
relations, and some but not all of the handling that we currently have
for appendrels, since they are a mix of the two.  Refactor a little
bit so that the necessary bits of logic are exposed as separate
functions.

Ashutosh Bapat, reviewed and tested by Rajkumar Raghuwanshi and
by me.

Discussion: http://postgr.es/m/CAFjFpRfqotRR6cM3sooBHMHEVdkFfAZ6PyYg4GRZsoMuW08HjQ@mail.gmail.com