Persistence whilst awaiting initial connection

[MattBrittan]

Adds the options ConnectRetry and ConnectRetryInterval along with the code to implement them and enable messages to be stored whilst the connection is in progress. The aim is to enable Publish() to be called before a connection is established (the message will get added to the store and sent eventually).

This is useful in situations where the comms are unreliable and the system may need to shutdown for periods to reduce power use. With this change there is no need to write separate code to store messages that are generated before the connection comes up.

The majority of changes should only apply if the new flag is set to true; about the only change that will affect all users is moving the store open and set status to connect outside of the go routine (needed because a user could call connect() and then publish() immediately with this new option enabled).
There are also a few more locks to align connect/reconnect - I was going to separate out the connect code so that the same code could be used for connect & reconnect but figured that you would prefer a smaller change.

closes #332

[alsm]

Thanks for the work on this, I'll take a closer look over the next day or two before deciding. I had avoided allowing Publish before connect in case you never successfully connect, but in that case is it any worse to store messages that will never be delivered, probably not.

[MattBrittan]

I had avoided allowing Publish before connect in case you never successfully connect

That was one of my concerns too - I deal with the possibility with something like the below (as a go routine) in my application code. This allows the end user to ascertain if there is an issue when performing initial setup.

for {
	done := token.WaitTimeout(1 * time.Minute)
	if done {
		if token.Error() != nil {
			***Log Error Message.... Happens if Disconnect is called before connection completes 
		} else {
			***Log Info Message indicating successful connection
		}
		return // token all done - nothing more will happen
	}
	*** Log info saying connect still in progress
}