http: forward-port missing bits from 01e2920

Forward-port the comments from commit 01e2920 (v0.10) to the master
branch. Everything else from that patch already exists in master.

It didn't merge cleanly because lib/http.js has been split up in
several files.

lib/_http_incoming.js

@@ -102,6 +102,9 @@ IncomingMessage.prototype._read = function(n) {
 };
 
 
+// It's possible that the socket will be destroyed, and removed from
+// any messages, before ever calling this.  In that case, just skip
+// it, since something else is destroying this connection anyway.
 IncomingMessage.prototype.destroy = function(error) {
   if (this.socket)
     this.socket.destroy(error);

lib/_http_outgoing.js

@@ -93,6 +93,9 @@ OutgoingMessage.prototype.setTimeout = function(msecs, callback) {
 };
 
 
+// It's possible that the socket will be destroyed, and removed from
+// any messages, before ever calling this.  In that case, just skip
+// it, since something else is destroying this connection anyway.
 OutgoingMessage.prototype.destroy = function(error) {
   if (this.socket)
     this.socket.destroy(error);