[Bugfix] Raw sockets not being closed when Tracer is dropped

[johnyburd]

As a result of a moving a reference to the sockets into an endless task, the raw sockets for the tracer will not be closed even after the Tracer is dropped. This will eventually result in using all of the file handles on the system if the Tracer is instantiated in a loop.

This PR fixes the issue by spawning the tasks on a JoinSet which automatically aborts tasks when it is dropped.