Add start time to pg_stat_activity

doc/src/sgml/func.sgml

doc/src/sgml/monitoring.sgml

doc/src/sgml/runtime.sgml

src/backend/catalog/heap.c

src/backend/catalog/namespace.c

src/backend/commands/cluster.c

src/backend/commands/sequence.c

src/backend/commands/tablecmds.c

src/backend/executor/execMain.c

src/backend/libpq/hba.c

src/backend/postmaster/pgstat.c

src/backend/utils/adt/nabstime.c

src/backend/utils/adt/pgstatfuncs.c

src/backend/utils/error/elog.c

src/bin/initdb/initdb.sh

src/bin/pg_dump/pg_backup_tar.c

src/include/catalog/catversion.h

src/include/catalog/pg_proc.h

src/include/pgstat.h

src/test/regress/expected/rules.out