Investigate potential re-use of EVP_CIPHER_CTX

The encrypt and decrypt path each appear to [initialize an EVP_CIPHER_CTX](https://github.com/aws/aws-encryption-sdk-c/blob/6005f41bf70ab9ef51139f80756cd30cde270d85/source/cipher.c#L435) twice using the same content key, which may represent a redundant AES key schedule. Re-using this context could avoid a redundant key schedule. This would likely require some refactoring of the aws_cryptosdk_session and the encrypt/decrypt paths, as well as the corresponding proofs.

Start at source/cipher.c around the linked EVP_CIPHER_CTX initialization and trace how the aws_cryptosdk_session encrypt and decrypt paths manage the content key. Determine whether the context can be reused safely, then assess the corresponding proofs; the investigation is done when the refactoring scope and its proof implications are established.

Written by the indexing model from the issue text.