Periodic re-registration of transports with TPD

[0pcom]

The underlying / related issue is stated here

skycoin/skywire#1715

and here

https://github.com/skycoin/skywire-service-discovery/issues/1

There are without a doubt dead entries for transports in transport discovery

This issue should be handled similarly to the ticket in service discovery.

Expect periodic re-registration of a transport at specified interval or consider the transport inactive.