Add reaper to remove expired sessions

The reaper is a background server that periodically (every 60s)
finds and clears old sessions that haven't been actively retrieved
or updated for a specified amount of time (TTL). The TTL can be
change by settning {ttl, Seconds} in your app.config.

Notice that in the current implementation, the TTL is more
of an approximation than a hard limit; since the reaper
checks for expired sessions every 60 seconds, a session
may outlive its TTL by up to 59 seconds.