pgsql: Simplify code by getting rid of SPI_push, SPI_pop, SPI_restore_c

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>

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/1833f1a1c3b0e12b3ea40d49bf11898eedae5248

Modified Files
--------------
doc/src/sgml/spi.sgml                | 180 ++++++++-----------------------
src/backend/executor/spi.c           | 200 +++++++++--------------------------
src/backend/utils/adt/xml.c          |   6 --
src/backend/utils/cache/plancache.c  |  13 ---
src/backend/utils/fmgr/fmgr.c        |  48 +--------
src/include/executor/spi.h           |  12 ++-
src/pl/plperl/plperl.c               |  78 --------------
src/pl/plpgsql/src/pl_exec.c         |  21 ----
src/pl/plpython/plpy_exec.c          |   2 -
src/pl/plpython/plpy_spi.c           |  13 ---
src/pl/plpython/plpy_subxactobject.c |   7 --
src/pl/tcl/pltcl.c                   |  18 ----
12 files changed, 105 insertions(+), 493 deletions(-)