ISSUE-131: more than 3 topics blocks async operations

[sijie]

Original Issue: apache#131

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If we have more than 3 active topics Pulsar blocks all async operations such as readdir, readFile, http.get, etc. on NodeJS
So, I cannot read even static files, do HTTP requests, etc.

I prepared for you a pure NodeJS example with "Pulsar client.subscribe" and 4 topics here