Input parser strictness for control frames and DoS

[lpeterse]

I'm currently trying to find a solution for the issues #60 and #149. They are more serious than one might expect: Whenever a server blocks on receive a client can send an arbitrary large message and easily exhaust the servers memory.

While reviewing the code I noticed that RFC 6455 (section 5.5) mandates that control frames are never fragmented and their payload does not exceed 125 bytes. I found that this is not enforced by the current implementation allowing an attacker to even bring down a server with a PING or CLOSE frame.

I prepared a PR enforcing these properties in the decoder.