[GOAL2-784] Fix random network unit test failures

[tsachiherman]

Summary

Our existing websocket library would fail to flush the first few messages, as long as the messages accumulated size is below 4096 bytes.

Explanation

The websocket library is creating a thread that is responsible for flushing the pending buffer content. When a write operation is complete, the flushing thread is being woken up to flush the buffer content.
The issue is that upon startup, the connection might get created, followed by several messages being sent. If that happens before the flushing thread start to execute, the flushing thread might miss the signal which indicates it needs to flush the outgoing buffer content.

In the context of our unit tests, where we send two small messages and wait until they reach the other socket, it being expressed as a flaky test.

Solution

The first for this is pretty straight forward; on the flushing thread, just flush the content of the buffer upon startup without waiting for a signal. Then, wait for the signal which would handle the subsequent iteration.

[Vervious]

Fixes #74