Prevent leakage of SPI tuple tables during subtransaction abort.

plpgsql often just remembers SPI-result tuple tables in local variables,
and has no mechanism for freeing them if an ereport(ERROR) causes an escape
out of the execution function whose local variable it is.  In the original
coding, that wasn't a problem because the tuple table would be cleaned up
when the function's SPI context went away during transaction abort.
However, once plpgsql grew the ability to trap exceptions, repeated
trapping of errors within a function could result in significant
intra-function-call memory leakage, as illustrated in bug #8279 from
Chad Wagner.

We could fix this locally in plpgsql with a bunch of PG_TRY/PG_CATCH
coding, but that would be tedious, probably slow, and prone to bugs of
omission; moreover it would do nothing for similar risks elsewhere.
What seems like a better plan is to make SPI itself responsible for
freeing tuple tables at subtransaction abort.  This patch attacks the
problem that way, keeping a list of live tuple tables within each SPI
function context.  Currently, such freeing is automatic for tuple tables
made within the failed subtransaction.  We might later add a SPI call to
mark a tuple table as not to be freed this way, allowing callers to opt
out; but until someone exhibits a clear use-case for such behavior, it
doesn't seem worth bothering.

A very useful side-effect of this change is that SPI_freetuptable() can
now defend itself against bad calls, such as duplicate free requests;
this should make things more robust in many places.  (In particular,
this reduces the risks involved if a third-party extension contains
now-redundant SPI_freetuptable() calls in error cleanup code.)

Even though the leakage problem is of long standing, it seems imprudent
to back-patch this into stable branches, since it does represent an API
semantics change for SPI users.  We'll patch this in 9.3, but live with
the leakage in older branches.