manager: be more aggressive about purging http sessions.

manager: be more aggressive about purging http sessions.

If we find that n_max (currently hard wired to 1) sessions were purged,
schedule the next purge for 1ms into the future rather than 5000ms (as
per current).  This way we will purge up to 1000 sessions per second
rather than 1 every 5 seconds.

This mitigates a build-up of sessions should http sessions gets
established faster than 1 per 5 seconds.

Change-Id: I9820d39aa080109df44fe98c1325cafae48d54f5
Signed-off-by: Jaco Kroon <jaco@uls.co.za>