ban_lurker_age, ban_lurker_sleep

[Nils Goroll]

I've just pushed two minor improvements on the documentation, but I'd suggest
two changes

* ban_lurker_age:

By holding off the ban lurker for a bit,
- we increase the likelihood of removing duplicate bans before spending time
  on testing them and
- increase the number of bans we test against each object in a single go

but we also increase the likelihood of bans requiring request-time evaluation,
which is bad for latency.

I cannot present generic real world numbers, because I have only looked at one
real life application today. This application issues bans in bursts which it
fires within a couple of seconds.

Are there any other applications which are likely to issue duplicate bans over
longer periods of time? Otherwise I'd suggest that we lower the default to 5
seconds.

* ban_lurker_sleep

This is good for holding off the ban lurker after ban_lurker_batch, but IMHO
it's really bad that the param is also used for the sleep time at lock contention.

I strongly suggest that we make these two different parameters. 10ms default
sounds sensible, but people may be tempted to increase ban_lurker_sleep to
seconds timeframes and that could really hurt in the contention case.

Thx, Nils