Store the purger runtime measurements in key-value storage instead of state

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Problem/Motivation

Like the queue statistics in #3635585: Store the queue statistics in key-value storage instead of state, the runtime measurements of purgers are stored in state. A purger that measures its runtime, such as the generic HTTP purger, changes its measurement after most successful processing runs. Each of those writes invalidates the cached state entry that every request loads.

Proposed resolution

Store the measurements in a key-value collection, and move the current values in a post-update function. Keep the deprecated $state property and constructor argument with the same backwards compatibility layer as #3635585: Store the queue statistics in key-value storage instead of state, and rename its state wrapper so that both trackers can use it.

The wrapper was already released in 3.8, but it's marked @internal and @deprecated, so people are not supposed to use it. This should not be considered a breaking change.

Remaining tasks

• Validate the proposed solution
• Create a MR
• Review