ROS1: Work needed to support native ROS1 nodes

[ssnover]

Did a little spelunking through ros/ros_comm some time ago and I think the things needed to achieve basic pubsub on ROS1 are:

• Start generating md5sums for message types and add a method to get that md5sum
• For publishers:
  • Host an XMLRPC on a TCP port
  • Call registerPublisher method on rosmaster XMLRPC API.
  • Handle connection headers from subscribers which want to listen by responding with header response
  • Then send rosmsg encoded data on the stream above
• For subscribers
  • Request the XMLRPC URIs from rosmaster for a given topic
  • Listen for new publishers registered on an XMLRPC URI
  • Make TCPROS connections to publishers (first with a connection header), wait for connection header response
  • Handle incoming rosmsg encoded data.

Relevant docs:

• TCPROS docs
• rosmsg wire format
• rosmaster XML RPC handling code
• roscpp xmlrpc code