use rwmutex instead of mutex

[modular-magician]

mutex was added in GoogleCloudPlatform/magic-modules#5722 to fix concurrent map writes error in new vcr test. However it still constantly ran into concurrent map read and write error, as the previous only add the lock when write data to the map. Therefore, in this PR

• Add the locking mechanism whenever the map is accessed (including both read-from and write-to)
• Use rwmutex instead of mutex for better efficiency (which enables concurrent read access, just not concurrent read/write or write/write access)

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Derived from GoogleCloudPlatform/magic-modules#5732