Fix lock up after first connection.

Prevent reuse of \fpoirotte\Pssht\Handlers\USERAUTH\REQUEST instances which
resulted in a lock up after the first successful connection.
Add checks on key re-exchange parameters.
Fix UMAC tag generation.

Fixes an issue with UMAC tag generation when the nonce has leading null bytes.
Add getKeySize() method to MACInterface.

This method is used to distinguish between key size and generated MAC size.
This is useful for algorithms where the two are different (eg. UMAC).