Use only pull-streams, reduce memory usage and improve perf by a **lot**

long story short: libp2p-mplex has been around for a while, longer than our migration for pull-streams. Things work now but we know that using Readable Streams in the browser is a huge memory inefficiency and might cause bugs that we are not even aware right now.

Now that #68 #72 happened. We should upgrade the whole module to drop Readable Streams entirely.