[FIX] res.users: don't delay login() if user row is currently locked

[FIX] res.users: don't delay login() if user row is currently locked

Any running transaction that has created or altered
a record that contains a FK to the user is automatically
holding an SharedRowLock that prevents login() from
updating the last login date. As we cannot delay the
login() until all such transactions are finished,
it's simpler to try to get the lock and if that
fails, we skip the login date update for once,
no big deal.
A typical candidate for this situation is the
Admin user, locked out by long running cron
jobs that touch rows as Admin, hence holding
a ShareRowLock to res.users ID 1 because of the
many create_uid/write_uid foreign keys.

lp bug: https://launchpad.net/bugs/713216 fixed

bzr revid: odo@openerp.com-20120213183207-45ptopqm0szritgx