Performance improvement: reduce savepoints in function

[strk]

It's been reported that savepoints/EXCEPTION (subtransactions) are particularly expensive. The topology code make heavy use of them. Switching back may give _a_lot_ of performance benefit.

Quoting from #postgresql:

 < andres> (like two orders of magnitude or so)
 < andres> improving subtxn performance is on the roadmap, but one partial patch was reverted again (incomplete, broken)
 < andres> It also doesn't scale linearly with multiple subtxn in one txn

... may need a 2.0.1 milestone

[strk]

Reference about error trapping being "significantly more expensive":  http://www.postgresql.org/docs/8.4/interactive/plpgsql-control-structures.html#PLPGSQL-ERROR-TRAPPING

[strk]

Enhancements, later