Filter on monitor name

[reubano]

From 06ff724#commitcomment-12270378 and 06ff724#commitcomment-12271345

To quote the readme:

You can even run several monitor servers in several datacenters to get average response time.
In that case, make sure you set a different monitor.name setting for all monitor servers to be
able to tell which server make a particular ping.

Yea possible, but the monitors won't ping at the same time.
So we need to create checksNeedingPollForMonitor to get the checks that a specific monitor needs
to check. Yea, yours has it too. We should also filter on monitor name