This takes advantage of the infrastructure introduced by commit
81c5e46c to greatly reduce the
likelihood that two different queries will end up with the same query
ID.  It's still possible, of course, but whereas before it the chances
of a collision reached 25% around 50,000 queries, it will now take
more than 3 billion queries.

Backward incompatibility: Because the type exposed at the SQL level is
int8, users may now see negative query IDs in the pg_stat_statements
view (and also, query IDs more than 4 billion, which was the old
limit).

Patch by me, reviewed by Michael Paquier and Peter Geoghegan.

Discussion: http://postgr.es/m/CA+TgmobG_Kp4cBKFmsznUAaM1GWW6hhRNiZC0KjRMOOeYnz5Yw@mail.gmail.com