This makes us Suite B compliant and saves 32 bytes per frame. The
AES/GCM implementation refuses to decrypt the frame header before
checking the MAC, so we have to use AES/CTR to peek at the header. The
header is still covered by the MAC, and we still check it after peeking!

CryptoComponentImpl needs some unit tests.

This matches the security level of AES-256 according to NSA Suite B. To
better comply with Suite B we should replace the combination of CTR mode
and HMAC with GCM, which would reduce the MAC size from 48 to 16 bytes.

consequent distinction between segments and frames.