Be lazier about partition tuple routing.

Be lazier about partition tuple routing.

It's not necessary to fully initialize the executor data structures
for partitions to which no tuples are ever routed.  Consider, for
example, an INSERT statement that inserts only one row: it only cares
about the partition to which that one row is routed.  The new function
ExecInitPartitionInfo performs the initialization in question only
when a particular partition is about to receive a tuple. This includes
creating, validating, and saving a pointer to the ResultRelInfo,
setting up for speculative insertions, translating WCOs and
initializing the resulting expressions, translating returning lists
and building the appropriate projection information, and setting up a
tuple conversion map.

One thing that's not deferred is locking the child partitions; that
seems desirable but would need more thought.  Still, testing shows
that this makes single-row inserts significantly faster on a table
with many partitions without harming the bulk-insert case.

Amit Langote, reviewed by Etsuro Fujita, with a few changes by me

Discussion: http://postgr.es/m/8975331d-d961-cbdd-f862-fdd3d97dc2d0@lab.ntt.co.jp