Fix regression in parallel planning against inheritance tables.

Fix regression in parallel planning against inheritance tables.

Commit 51ee6f3160d2e1515ed6197594bda67eb99dc2cc accidentally changed
the behavior around inheritance hierarchies; before, we always
considered parallel paths even for very small inheritance children,
because otherwise an inheritance hierarchy with even one small child
wouldn't be eligible for parallelism.  That exception was inadverently
removed; put it back.

In passing, also adjust the degree-of-parallelism comptuation for
index-only scans not to consider the number of heap pages fetched.
Otherwise, we'll avoid parallel index-only scans on tables that are
mostly all-visible, which isn't especially logical.

Robert Haas and Amit Kapila, per a report from Ashutosh Sharma.

Discussion: http://postgr.es/m/CAE9k0PmgSoOHRd60SHu09aRVTHRSs8s6pmyhJKWHxWw9C_x+XA@mail.gmail.com