Deduplicate muxer for a given same stream. (#5)

* docs: add documentation for iterating over channels

* fix: deduplicate protomux instances for the same stream

creating multiple channels on multiple muxers for the same stream closes
channels accidently see
[here](https://gist.github.com/Nazeh/a3f1d24b597913303afcb6c568f4b042)

* fix: deduplicate muxers on the same stream

opening multiple channels on multiple muxers causes channels to either
close or reset, see
[here](https://gist.github.com/Nazeh/a3f1d24b597913303afcb6c568f4b042)

* use userData

index.js

@@ -245,6 +245,8 @@ class Channel {
 
 module.exports = class Protomux {
   constructor (stream, { alloc } = {}) {
+    if (stream.userData === null) stream.userData = this
+
     this.isProtomux = true
     this.stream = stream
     this.corked = 0
@@ -271,6 +273,7 @@ module.exports = class Protomux {
   }
 
   static from (stream, opts) {
+    if (stream.userData && stream.userData.isProtomux) return stream.userData
     if (stream.isProtomux) return stream
     return new this(stream, opts)
   }

test.js

@@ -345,6 +345,52 @@ test('alias', function (t) {
   bp.addMessage({ encoding: c.string }).send('hello world')
 })
 
+test('deduplicate muxers', function (t) {
+  const sa = new SecretStream(true)
+  const sb = new SecretStream(false)
+
+  replicate({ stream: sa }, { stream: sb })
+
+  const a = Protomux.from(sa)
+  const foo = a.createChannel({
+    protocol: 'foo',
+    onopen () { t.pass('a remote opened') }
+  })
+
+  foo.open()
+
+  foo.addMessage({
+    encoding: c.string,
+    onmessage (message) { t.is(message, 'hello foo') }
+  })
+
+  const bfoo = Protomux.from(sb).createChannel({ protocol: 'foo' })
+
+  // Another Protomux instance for another protocol
+  const a2 = Protomux.from(sa)
+  const bar = a2.createChannel({
+    protocol: 'bar',
+    onopen () { t.pass('a remote opened') }
+  })
+
+  bar.open()
+
+  bar.addMessage({
+    encoding: c.string,
+    onmessage (message) { t.is(message, 'hello bar') }
+  })
+
+  const bbar = Protomux.from(sb).createChannel({ protocol: 'bar' })
+
+  t.plan(4)
+
+  bfoo.open()
+  bfoo.addMessage({ encoding: c.string }).send('hello foo')
+
+  bbar.open()
+  bbar.addMessage({ encoding: c.string }).send('hello bar')
+})
+
 function replicate (a, b) {
   a.stream.rawStream.pipe(b.stream.rawStream).pipe(a.stream.rawStream)
 }