Refactor to abstract away the p2p request handler from mailserver (#34)

tests/v1/test_waku_mail.nim

@@ -102,7 +102,7 @@ procSuite "Waku Mail Client":
       var dummy: Hash
       traceAsyncErrors peer.p2pRequestComplete(dummy, dummy, cursor)
 
-    simpleServer.enableMailServer(customHandler)
+    simpleServer.registerP2PRequestHandler(customHandler)
     check client.setPeerTrusted(simpleServerNode.id)
     var request: MailRequest
     var symKey: SymKey

waku/protocol/v1/waku_mail.nim

@@ -79,8 +79,7 @@ proc p2pRequestHandler(peer: Peer, envelope: Envelope) =
   # Mail server p2p request implementation
   discard
 
-proc enableMailServer*(node: EthereumNode, customHandler: P2PRequestHandler) =
-  node.protocolState(Waku).p2pRequestHandler = customHandler
-
 proc enableMailServer*(node: EthereumNode) =
-  node.protocolState(Waku).p2pRequestHandler = p2pRequestHandler
+  # TODO: This could become part of an init call for an actual `MailServer`
+  # object.
+  node.registerP2PRequestHandler(p2pRequestHandler)

waku/protocol/v1/waku_protocol.nim

@@ -640,6 +640,10 @@ proc configureWaku*(node: EthereumNode, config: WakuConfig) =
   ## of the settings are only communicated at peer handshake.
   node.protocolState(Waku).config = config
 
+proc registerP2PRequestHandler*(node: EthereumNode,
+    customHandler: P2PRequestHandler) =
+  node.protocolState(Waku).p2pRequestHandler = customHandler
+
 proc resetMessageQueue*(node: EthereumNode) =
   ## Full reset of the message queue.
   ##