Optimize concurrent collection's shrink logic

[wenbingshen]

Motivation

Optimize concurrent collection's shrink and clear logic.

This PR was releated to bookkeeper's PR：apache/bookkeeper#3417

Modifications

1. Reduce the repeated Arrays.fill in the clear process
2. When capacity is already equal to initCapacity,rehash should not be executed
3. Reduce the rehash logic in the clear process
4. Shrinking must at least ensure initCapacity, so as to avoid frequent shrinking and expansion near initCapacity, frequent shrinking and expansion, additionally opened arrays will consume more memory and affect GC.

Documentation

Need to update docs?

• doc-not-needed

[wenbingshen]

/pulsarbot run-failure-checks