Removed extra comments
Added HostLeave Event

When a host loose its connection host_tracker sends a ARP ping.
If there is no answer for the ping, host_tracker removes the
host entry. Now, when it happens host_tracker notifies topology
that removes the host entity and trigger the HostLeave event.
host_tracker refactoring

Organized imports;
Remove unnecessary imports;
DocStrings corrections;
Switched class constructors to super;
Adjusted code based on the documentation code design;