[Patch] Add a reset_computed_values function in pg_stat_statements

Hello

pg_stat_statements is a great tool to track performance issue in live
databases, especially when adding interfaces like PoWA on top of it.
But so far, tools like PoWA can not track the min_time, max_time, mean_time
and sum_var_time of queries : these statistics are cumulated over time,
fetching points in time would be of little to no use, especially when looking
at the impact of a DDL change.
This patch thus introduces a simple pg_stat_statements_reset_computed_values
function that reset the computed statistics, leaving all other information
alive, thus allowing the aforementioned scenario.

Regards

Pierre Ducroquet