Sample values for pg_stat_statements

Often when looking through pg_stat_statements, it would be nice to have
some sample values for the constants and parameters.  This patch
implements that by taking the values from the first execution of the
normalized query.

To keep things reasonable, there is a limit on how big the parameters
can be.

This patch is based off of 5303ffe71b.
-- 
Vik Fearing                                          +33 6 46 75 15 36
http://2ndQuadrant.fr     PostgreSQL : Expertise, Formation et Support