Bug 36149: Unset userenv from middleware

Bug 36149: Unset userenv from middleware

The userenv (logged in user's info) are stored in
$C4::Context->context->{activeuser}, which persists in plack worker's
memory.
It's really bad in theory as we are not cleaning it before or after the
HTTP request, but only when set_userenv is called (what we are doing
commonly in C4::Auth::get_template_and_user).
If C4::Context->userenv is called before set_userenv we should get undef,
not the userenv from the previous request!
In practice this should not be a problem, but well... who really knows?

This patch suggests to have a middleware to deal with removing the
userenv at the beginning of each request (maybe it should be after, right? - FIXME).

To test:
1 - Edit /etc/koha/sites/kohadev/koha-conf.xml to set <plack_workers>1</plack_workers>
2 - Edit about.pl  and add a line after: CGI->new:
    warn Data::Dumper::Dumper( C4::Cointext->userenv() );
3 - tail -f /var/log/koha/kohadev/*.log
4 - View about.pl in staff interface, should get a "somethign's wrong" warning
5 - Reload, you get current user info
6 - Open an incognito tab, sign in as a different user and click some stuff
7 - Reload about.pl in other window
8 - You get the opac user info
9 - Apply patch
10 - Edit /etc/koha/sites/kohadev/plack.psgi and add the middleware after "RealIP":
     enable "+Koha::Middleware::UserEnv";
11 - Restart all
12 - Reload about.pl - you get a "Something's wrong" warning
13 - Click things in opac on incognito window
14 - Reload about.pl  - only "Something's wrong" - you no longer see any user info

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>