CAMEL-21888: camel-core - Compute cache changes size

[essobedo]

Fix for https://issues.apache.org/jira/browse/CAMEL-21888

Motivation

Iterating over all the changes to get size is very costly in case of a big cache size. Using a counter to have the size of changes was initially avoided for consistency issues reason, but the addition lately of a ReadWriteLock allows us to put it back

Modifications:

• Avoid an O(N) to get the size of the changes by counting them using an AtomicInteger.

[github-actions]

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