Extract Class A End Devices into their own class

[peggyopal]

The title says it all!

This way it would make it really easy to create new classes for Class B and C devices to extend from a base EndDeviceLorawanMac class.

[DvdMgr]

I merged the PR relative to this issue, I feel like we can close this unless you'd like to have further discussion!

[peggyopal]

I have to agree with you!

Also, I'm sorry about the error that happened. I thought I combed through the logs enough, apparently I did not! Would you mind sharing how you found it? :)

[DvdMgr]

I ran network-server-example and saw that the second device, which should require an ACK, tried to send the message 8 times. The Network Server correctly received the message, and the Gateway was sending replies, however I saw from the log that the Close(First/Second)ReceiveWindow calls happened right after Open(First/Second)ReceiveWindow, and thus did not allow the PHY to lock on the incoming packet. I added logging to show the value of m_receiveWindowDurationInSymbols, which is used to compute the delay at which to close the window, and saw that it was zero, thus explaining the non-existent delay between opening and closing of the receive windows. It was enough to initialize the variable to get it to work correctly!