fix: separate auth cache and session pool to prevent race condition

[taka-oyama]

Coming from googleapis/google-cloud-php#6703 (comment)

Auth does not lock the cache file, so it's possible for a race condition to occur.

Below is an example of how that may happen.

1. Process 1: Auth opens cache
2. Process 2: Session Pool opens cache
3. Process 2: Session Pool creates a session and saves (file is written)
4. Process 1: Auth writes to cache (file is overwritten and session created at 3. no longer exists in cache)

• CHANGELOG

Must be backported to 6.0.