Bug in concurrency control in temporary GiST indexes

While hacking on the GiST insertion algorithm, I noticed a bug with 
temporary GiST indexes. The scan algorithm uses LSNs to detect a 
concurrent page split, but for a temporary index, we just use a constant 
LSN on the assumption that there can't be anyone else modifying the 
index concurrently. But that assumption is not true. While a temporary 
index can't be modified by other backends, it's entirely possible to 
insert new tuples to the index within the same backend while a scan is 
in progress.

Here's a test case, using btree_gist. If you modify it to use a 
non-temporary table, it works fine, but with a temporary table the 2nd 
SELECT misses some rows, when new rows are inserted in the middle of the 
scan.

Operations on temporary tables are not WAL-logged, so we don't have LSNs 
to use, but we just need a monotonically increasing sequence of numbers 
for this purpose. Attached patch provides a function to generate such 
fake LSNs.

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