Thread pool state health check

[Andrus Adamchik]

Let's implement a few health checks based on the existing Jetty thread pool metrics:

bq.jetty.threadPool.utilization

bq.jetty.threadPool.queuedRequests

WARNING and CRITICAL thresholds will be configured as follows:

jetty:
  health:
    queuedRequestsThreshold: "5 10"
    poolUtilizationThreshold: "0.5 0.95"