Improve latency detectors using percentile

[xp-1000]

especially for alerting purpose it seems to be a good improvement to use percentile on latency detectors (and may be others).

example for ELB:

A = data('Latency', filter=filter('AvailabilityZone', '*') and filter('LoadBalancerName', '*') and filter('stat', 'mean'), extrapolation='last_value', maxExtrapolations=5).sum(by=['LoadBalancerName']).publish(label='A', enable=False)
B = (A).min().publish(label='B')
C = (A).percentile(pct=90).publish(label='C')
D = (A).max().publish(label='D')