[RECEIVE strategy] Add support for receiving topology from nodes

[nemesifier]

Right now the collector must be able to reach topology urls in order to collect data and visualize the topology.

This is not always easy and in some cases not possible at all due to NAT or lack of public IP availablity. In those cases users have to develop scripts that send the topology to a server with a public IP and the collector must query that server.

To alleviate this pain, it would be easier to send the topology data directly to the collector.

• modify Topology model
• implement simple receive strategy (ttl == 0)
• add view to API
• implement multiple receive strategy (ttl > 0)
• improve Topology admin form (show relevant fields according to strategy)
• find a better name for ttl

[nemesifier]

hellyeah! 👍