Split Main & Streaming socket

[dsienkiewicz]

Solves #8 , #10 and #11 .

Problem & solution

1. Public API should return result tuples.
   Whenever public API functions are called, result should be formed as {:ok, struct()} or {:error, %Error{}} tuple.
2. StreamingSocket module now is useable, which means it defines callbacks, which should be implemented on client side to receive streamed messages. It means there's no longer juggling of messages between streaming socket process and connection process - as connection process is no longer needed.
   It also means the management of main socket & streaming socket is up to client.
3. With removal of connection process there's no issue with heavy init calls.