Add contrib/pg_stat_statements for server-wide tracking of statement execution

contrib/Makefile

contrib/README

contrib/pg_stat_statements/Makefile

contrib/pg_stat_statements/pg_stat_statements.c

contrib/pg_stat_statements/pg_stat_statements.sql.in

contrib/pg_stat_statements/uninstall_pg_stat_statements.sql

doc/src/sgml/contrib.sgml

doc/src/sgml/filelist.sgml

doc/src/sgml/pgstatstatements.sgml

src/backend/postmaster/postmaster.c

src/include/pgstat.h