[PATCH] A hook for session start

Hi,

Currently, PostgreSQL doen't have a hook triggered at session
start. Although we already have ClientAuthentication_hook,
this is triggered during authentication, so we can not
access the database. 

If we have a hook triggerd only once at session start, we may
do something useful on the session for certain database or user.

For example, one of our clients wanted such feature. He wanted
to handle encription for specific users, though I don't know
the detail.

The attached patch (session_start_hook.patch) implements such
hook. 

Another patch, session_start_sample.patch, is a very simple
example of this hook that changes work_mem values for sessions
of a specific database. 

I would appreciate hearing your opinion on this hook.

Regards,

-- 
Yugo Nagata <nagata@sraoss.co.jp>