Health check convenience classes

[thomasleese]

This PR adds a few useful health check base classes which can be used as the basis for more specific application checks. It also improves the documentation and adds the ability to disable/enable health checks during the course of an applications lifetime.

The base classes added are:

• Threshold check which compares a value (or a set of values) with a warning and a critical threshold
• Sidekiq queue size check which compares the size of individual queues with thresholds
• Sidekiq queue latency check which compares the latency of individual queues with thresholds
• Generic Sidekiq queue check which makes it easy to have a threshold check which gets applied on each queue individually

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