Simplify code by getting rid of SPI_push, SPI_pop, SPI_restore_connection.

The idea behind SPI_push was to allow transitioning back into an
"unconnected" state when a SPI-using procedure calls unrelated code that
might or might not invoke SPI.  That sounds good, but in practice the only
thing it does for us is to catch cases where a called SPI-using function
forgets to call SPI_connect --- which is a highly improbable failure mode,
since it would be exposed immediately by direct testing of said function.
As against that, we've had multiple bugs induced by forgetting to call
SPI_push/SPI_pop around code that might invoke SPI-using functions; these
are much harder to catch and indeed have gone undetected for years in some
cases.  And we've had to band-aid around some problems of this ilk by
introducing conditional push/pop pairs in some places, which really kind
of defeats the purpose altogether; if we can't draw bright lines between
connected and unconnected code, what's the point?

Hence, get rid of SPI_push[_conditional], SPI_pop[_conditional], and the
underlying state variable _SPI_curid.  It turns out SPI_restore_connection
can go away too, which is a nice side benefit since it was never more than
a kluge.  Provide no-op macros for the deleted functions so as to avoid an
API break for external modules.

A side effect of this removal is that SPI_palloc and allied functions no
longer permit being called when unconnected; they'll throw an error
instead.  The apparent usefulness of the previous behavior was a mirage
as well, because it was depended on by only a few places (which I fixed in
preceding commits), and it posed a risk of allocations being unexpectedly
long-lived if someone forgot a SPI_push call.

Discussion: <20808.1478481403@sss.pgh.pa.us>

src/pl/plpython/plpy_subxactobject.c

@@ -7,7 +7,6 @@
 #include "postgres.h"
 
 #include "access/xact.h"
-#include "executor/spi.h"
 #include "utils/memutils.h"
 
 #include "plpython.h"
@@ -213,12 +212,6 @@ PLy_subtransaction_exit(PyObject *self, PyObject *args)
 	CurrentResourceOwner = subxactdata->oldowner;
 	pfree(subxactdata);
 
-	/*
-	 * AtEOSubXact_SPI() should not have popped any SPI context, but just in
-	 * case it did, make sure we remain connected.
-	 */
-	SPI_restore_connection();
-
 	Py_INCREF(Py_None);
 	return Py_None;
 }