Use unix time of the timestamp for data-order.

[meeusen]

See #485. Use the unix time to sort the timestamp column.

Testing done

Set timestamp of all resources and sort on timestamp column. Check that they are sorted chronologically

import org.jenkins.plugins.lockableresources.LockableResourcesManager

def d = 0;
LockableResourcesManager.get().getResources().each { resource ->
  resource.setReservedTimestamp(new Date().minus(d))
  d = d + 30    
}

[mPokornyETM]

LGTM

[mPokornyETM]

fix #485