Session and caching CMM helper functions
- Added API function to make session from keyring
- Added API function to make caching CMM from keyring
- Added "_from_cmm" to end of "aws_cryptosdk_caching_cmm_new" function name
- Changed "aws_cryptosdk_session_get_algorithm" to "aws_cryptosdk_session_get_alg_id"
- Fixed HKDF bug