[PDS-146088] Force-clear the pool if no evictions are happening

[Jolyon-S]

Goals (and why):
One symptom of PDS-146088 is that the client pool stops evicting. Force-clearing the pool if the eviction task has not even considered anything for eviction after some period of time is one approach that might work.

Implementation Description (bullets):
Clear the client pool if the last successful eviction run was over 15 minutes ago.

Testing (What was existing testing like? What have you done to improve it?):
No tests, which is perhaps dangerous, however I'm not sure what testing would or should look like here.

Concerns (what feedback would you like?):
As it stands, this PR is really bad - we need to also reset the last evicted time in the evictor, and the decomp is pretty wacky. I just wanted to get the concept down before I forgot.

Where should we start reviewing?:
CCPC.

Priority (whenever / two weeks / yesterday):
ASAP - we should get an RC out soon for internal testing.

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Type

• Feature
• Improvement
• Fix
• Break
• Deprecation
• Manual task
• Migration

Description

Force-evict the Cassandra client pool if no eviction has occurred for a while (by default, 60 * timeBetweenConnectionEvictionRunsSeconds, which makes this duration 20 minutes).

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[jeremyk-91]

Broadly makes sense. Two small suggested changes, and another that's more for if we want to productionise this. With those I'm not opposed to an RC, we can cut that tomorrow.