Update Append's idea of first_partial_plan

Update Append's idea of first_partial_plan

It turns out that after runtime partition pruning, Append's
first_partial_plan does not accurately represent partial plans to run,
if any of those got pruned.  This could limit participation of workers
in some partial subplans, if other subplans got pruned.  Fix it by
keeping an index of the first valid partial subplan in the state node,
determined at execnode Init time.

Author: David Rowley, with cosmetic changes by me.
Discussion: https://postgr.es/m/CAKJS1f8o2Yd=rOP=Et3A0FWgF+gSAOkFSU6eNhnGzTPV7nN8sQ@mail.gmail.com