Tuplestore should remember the memory context it's created in

With regards to this bug report:
http://archives.postgresql.org/pgsql-bugs/2009-12/msg00241.php

I think we should change tuplestore code so that callers of
tuplestore_put* don't need to switch to the correct memory context (and
resource owner, after this patch) before call. Instead,
tuplestore_begin_heap() should memorize the context and resource owner
used to create the tuplestore, and use that in tuplestore_put*
functions. AFAICS it is always a bug to be in a different memory context
in tuplestore_put* than in tuplestore_begin_heap(), so it would be more
robust to not put the burden on the callers.

Patch against CVS HEAD to do that and fix the reported bug attached. Now
that the tuplestore_put* switches to the right memory context, we could
remove that from all the callers, but this patch only does it for pl_exec.c.

Thoughts?

-- 
  Heikki Linnakangas
  EnterpriseDB   http://www.enterprisedb.com