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!

从 src/main/java/com/amazonaws/encryptionsdk/DefaultCryptoMaterialsManager.java 第 91 行附近开始，跟踪如何联系多个加密提供商。定义跨区域的提供商并行工作应如何运行，然后验证是否会并发联系各个提供商，并且对于所有已配置的提供商，加密仍能正确完成。

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