Add a thread that takes care of Peer presence.

Pipe advertisements are discovered only when the neighbor list is being populated. (page gets refreshed)
If other peers join the group, the existing peers will not see their pipe advertisements until a discovery request is made. (synchronize page is refreshed)
When a peer is disconnected, only the rdv peer it was connected to (unless it was itself a RDV) will detect it.
Also, currently pipe advertisements don't expire and stack up, even though the peers are offline for a long time.

Right now this is not a big problem, but when the periodical anti-entropy will be introduced, it will need a relatively up to date list of neighbors (pipe advertisements). However, it is causing problems when a peer wants to get the state of a group and has to go trough redundant and expired pipe advertisements.

Requested:

• Make pipe advertisements expire 6 minutes after being published.
• Add a thread that periodically discovers pipe advertisements and republishes the local pipe advertisement. It will run at a 5 minutes interval
• When a peer wants to disconnect, it will first republish its advertisement with expiration time set to 1 second, then it will continue with disconnect procedure. (not sure if it has the desired effect)

Result:

• Local cache of pipe advertisements will be up to date (5 minutes latency)
• Forced disconnected peers' pipe advertisements will expire in maximum 5 minutes because they were not renewed.
• Normal disconnected peer's pipe advertisements will expire immediately.