Parallel encryption across multiple regions

When multiple encryption providers are defined, can the [`DefaultCryptoMaterialsManager`](https://github.com/awslabs/aws-encryption-sdk-java/blob/master/src/main/java/com/amazonaws/encryptionsdk/DefaultCryptoMaterialsManager.java#L91) encrypt the additional providers in parallel rather than serially? Biggest use case for this would be having KMS providers defined in multiple disparate regions (Asia/Europe/US) and reducing the time waiting for each to contact them.

Thanks!

Start with src/main/java/com/amazonaws/encryptionsdk/DefaultCryptoMaterialsManager.java around line 91 and trace how multiple encryption providers are contacted. Define how parallel provider work should behave across regions, then verify that providers are contacted concurrently and that encryption still completes correctly for all configured providers.

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