Use percentage metric for thread pool utilization check

[Andrus Adamchik]

PercentValueRangeFactory is now available per bootique/bootique-metrics#27 . Let's switch Jetty metrics to percentages instead of doubles.

UPGRADE NOTES

In the following config:

jetty:
  health:
    poolUtilizationThreshold: "0.6 0.9"

fractions should be replaced with percentages:

jetty:
  health:
    poolUtilizationThresholds:
      warning: 60%
      critical: 90%