duplicate requests on CMM cache miss

### Problem:

While profiling an application we noticed duplicate request for encryption keys and decryption keys, typically after an DEK has just expired

### Solution:

The cause of this was on a miss used by expiration of an encryption DEK or a decryption DEK. In our case both were related to TTL
for a simple example - imaging 10 threads encrypting data, using the same keys. The key expires, and 10 threads, make 10 request to the MasterKey for 10 new encryption DEKs, which causes a little more latency, and cost (if the underlying KMS charges)

I think that the library can easily de-duplicate these requests (maybe as an option). We have done this in our application

Happy to contribute/colaborate on this but will need some steer from the maintainers

### Out of scope:

Is there anything the solution will intentionally NOT address?

[//]: # (NOTE: If you believe this might be a security issue, please email aws-security@amazon.com instead of creating a GitHub issue. For more details, see the AWS Vulnerability Reporting Guide: https://aws.amazon.com/security/vulnerability-reporting/ )