Remove TODOs to put into github issues

[lavaleri]

Issue #, if available: moved into issues:

• https://github.com/awslabs/aws-encryption-sdk-specification/issues/196
• https://github.com/awslabs/aws-encryption-sdk-specification/issues/197
• https://github.com/awslabs/aws-encryption-sdk-specification/issues/198
• https://github.com/awslabs/aws-encryption-sdk-specification/issues/199
• https://github.com/awslabs/aws-encryption-sdk-specification/issues/200
• https://github.com/awslabs/aws-encryption-sdk-specification/issues/201
• https://github.com/awslabs/aws-encryption-sdk-specification/issues/202
• https://github.com/awslabs/aws-encryption-sdk-specification/issues/203
• https://github.com/awslabs/aws-encryption-sdk-specification/issues/204
• https://github.com/awslabs/aws-encryption-sdk-specification/issues/205
• https://github.com/awslabs/aws-encryption-sdk-specification/issues/206
• https://github.com/awslabs/aws-encryption-sdk-specification/issues/207

captured in existing issues:

• https://github.com/awslabs/aws-encryption-sdk-specification/issues/12

Description of changes:

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