Teach contrib/pg_stat_statements to handle multi-statement commands better.

Make use of the statement boundary info added by commit ab1f0c82
to let pg_stat_statements behave more sanely when multiple SQL queries
are jammed into one query string.  It now records just the relevant
part of the source string, not the whole thing, for each individual
query.

Even when no multi-statement strings are involved, users may notice small
changes in the output: leading and trailing whitespace and semicolons will
be stripped from statements, which did not happen before.

Also, significantly expand pg_stat_statements' regression test script.

Fabien Coelho, reviewed by Craig Ringer and Kyotaro Horiguchi,
some mods by me

Discussion: https://postgr.es/m/alpine.DEB.2.20.1612200926310.29821@lancre