Fix TCP Connection data receive race condition

[dipinhora]

Prior to this commit, when we _accepted a new connection
or completed a connection via _event_notify there was a race
condition around reading data that the other party sent to us.

This existed because we didn't check to see if any data was
waiting on the socket for us already and instead only subscribed
to ASIO events for new read notifications. If the other party sent
us data after we signed up for ASIO events, we would successfully
receive notification and read the data. However, if the other
party sent us data before we signed up for ASIO events, we would
not read the data already on the socket and would wait for an ASIO
event telling us that we have data to read.

This commit fixes this race condition by ensuring that we try to
read from the socket whenever we _accept or complete a connection
via _event_notify prior to signing up for ASIO events. This
ensures we will not accidentally ignore any data that may already
be on the socket by the time we're ready to handle the connection.

[dipinhora]

I forgot to mention that this change makes the TCPMute, TCPUnmute and TCPThrottle test more reliable. They would sometimes previously fail because of the race condition around receiving data resolved by this commit.

[jemc]

The one failure in the CI matrix was a Travis blip, and unrelated to these changes. I'm merging.