Waku: rename capability to wkk

According to `doc/p2p.md` capability should be three letters.  I can't see this
in https://github.com/ethereum/devp2p/blob/master/rlpx.md#capability-messaging
though, but better safe than sorry.

This also dismabiguates and stays somewhat consistent in vocabulary:

Whisper -> Waku
whisper -> waku
shh     -> wkk

eth/p2p/rlpx_protocols/waku_protocol.nim

@@ -521,7 +521,7 @@ proc initMessage*(env: Envelope, powCalc = true): Message =
 proc hash*(msg: Message): hashes.Hash = hash(msg.hash.data)
 
 proc allowed*(msg: Message, config: WakuConfig): bool =
-  # Check max msg size, already happens in RLPx but there is a specific waku
+  # Check max msg size, already happens in RLPx but there is a specific wkk
   # max msg size which should always be < RLPx max msg size
   if msg.size > config.maxMsgSize:
     warn "Message size too large", size = msg.size
@@ -747,7 +747,7 @@ proc initProtocolState*(network: WakuNetwork, node: EthereumNode) {.gcsafe.} =
   asyncCheck node.run(network)
 
 p2pProtocol Waku(version = wakuVersion,
-                    rlpxName = "waku",
+                 rlpxName = "wkk",
                  peerState = WakuPeer,
                  networkState = WakuNetwork):