Adjust broker locking order slightly

[eapache]

I was doing an unrelated experiment with a config that generated a warning on
validation, and I ended up flooded with warnings because we now call Open on
every broker returned from the Client, and we were revalidating the config every
single time. One thing led to another and I made several other adjustments in
the same vein.

• Move the atomic AlreadyConnected check to before the config validation in
  Broker.Open. Config validation is expensive and can log, no point in doing
  it if we don't have to.
• Move all the atomic.StoreInt32 calls to after any of their related log
  messages. None of these were real problems, but it removes any possibility of
  out-of-order log messages due to unusual race conditions.
• Remove the double-check for AlreadyConnected after we've taken the full
  lock. Initially I just removed the log message since it's not something we
  need to log, but upon closer inspection I'm pretty sure this is dead code;
  anything that sets opened to 0 also sets conn to nil first, and does so
  while holding the lock.

@wvanbergen

[wvanbergen]

👍