This commit moves makeTestDB to db.go and exports it so that we'll be
able to use this function in other unit tests to make them testable with
etcd if needed.
This commit adds a new bucket to the database which is dedicated to
storing data pertaining to p2p related reachability for direct channel
counter parties. The data stored in this new bucket can be used within
heuristics when deciding to unilaterally close a channel due to
inactivity. Additionally, all known reachable IP addresses for a
particular LinkNode are to be stored and updated within the database in
order to facilitate the establishment of persistent connections to
direct channel counter parties.
