Fix plpgsql to release SPI plans when a function or DO block is freed.

This fixes the gripe I made a few months ago about DO blocks getting
slower with repeated use.  At least, it fixes it for the case where
the DO block isn't aborted by an error.  We could try running
plpgsql_free_function_memory() even during error exit, but that seems
a bit scary since it makes a lot of presumptions about the data
structures being in good shape.  It's probably reasonable to assume
that repeated failures of DO blocks isn't a performance-critical case.