implement query_start for pg_stat_activity

This patch implements the TODO item

- Add start time to pg_stat_activity

The implementation is straight-forward, except for two issues:

(1) The natural name for the backend function is
pg_stat_get_backend_activity_start(), which at 34 exceeds the
NAMEDATALEN limitation in previous releases of PostgreSQL. While that
limit has been raised, it seems a shame to me to not allow users to
manual lower it again -- so I renamed the function to
pg_stat_get_backend_qry_start(), and renamed the existing function that
fetches the query string from pg_stat_get_backend_activity() to
pg_stat_get_backend_qry() for consistency. If someone thinks that's the
wrong decision, let me know.

(2) I wasn't sure how to convert a struct timeval into a PostgreSQL
timestamp type, so I hacked something together involving ctime() and
timestamp_in(), but it seems clearly wrong. Can someone suggest how to
do this properly?

Cheers,

Neil

PS: Shouldn't the pg_stat_get_backend_xxx() functions be marked
"volatile"? (They're currently marked "stable")
--
Neil Conway <neilc(at)samurai(dot)com> || PGP Key ID: DB3C29FC