Buffer messages sending to the frontend before communication with frontend is established

[Kontinuation]

This fixes the race condition described in #7. The idea is pretty straightforward:

1. If send() is called before comm is established, the message to send is buffered in buffered_msgs
2. Once the comm is established, all buffered messages in buffered_msgs will be send sequentially to the frontend.

After applying this patch, we don't have to wait for comm to get ready before starting the SparkSession. We can start the SparkSession at any time and it will always work.

[derbeneviv]

@krishnan-r any chance this can be merged?
we tested it and it works for us