[FLINK-37166] Use concurrenthashmap in flink config manager

[gyfora]

Brief change log

Use concurrent hashmap to avoid reconciliation errors during initialization

Verifying this change

Unit test added

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changes to the CustomResourceDescriptors: no
• Core observer or reconciler logic that is regularly executed: yes

Documentation

• Does this pull request introduce a new feature? no

[gyfora]

cc @tomncooper

[tomncooper]

LGTM, out of interest what errors were you seeing?

[gyfora]

LGTM, out of interest what errors were you seeing?

ConcurrentModificationExceptions during reconciliation which lead to job reconciliation errors/retries