fixed block sizes for hmac-sha2-256 and hmac-sha2-512

These MACs where added in fdb891b842 but they didn't work for me after disabling md5&sha1. The server simply didn't respond anymore eventually resulting in a timeout. I debugged this and found two issues with the current code:

* both MACs have to use larger block/digest sizes, the previous values were the same that SHA1 had, but SHA2 needs 32 or 64 byte
* Additionally the KeyExchanger would only generate a 20 byte key when using diffie-hellman with sha1 as a KEX. For SHA2 larger keys are used, so the first message with a mac resulted in a "Corrupted MAC on input" error on the server. So the keys had to be resized like its already done for the Ciphers.