Implement new setup and teardown functions for the agent.

The setup function should:

1. Exit if any other agents are currently running on the same host
2. Establish a connection with the Monkey Island
3. Check if the agent has been marked for shutdown
4. Report appropriate telemetry if the agent is running on the Island
5. Upgrade from 32 -> 64bit agent if required (be careful not to recreate Mongo error in /api/telemetry #1059)
6. Configure the host's firewall
7. Start a tunnel
8. Construct a Puppet and Master
9. Register signal handlers (NOP for now, implemented in Implement signal handlers #1594)
10. Start the master

The teardown function should:

1. Stop the tunnel
2. Restore original firewall rules
3. Clean up the master

Tasks

• Refactor and clean up the agent setup/execute/teardown code. (0d) @ilija-lazoroski, @shreyamalviya