Add public locations storage

[rmatil]

Additionally to the user's own, domain protected storage for client locations,
a public one should also be introduced, so that arbitrary clients can join the same network.

On bootstrap, each client has to connect to one of the public locations, so that the DHT will be shared among all "online" clients. Otherwise, sharing would fail, since
the different users' clients would build separate DHTs and resolving locations of other users would be impossible.

[rmatil]

Since an end user has to specify a location of the client on startup, we
assume, he enters an IP of a client which is in the same network as the sharer.

Another approach, is to use an external service to fetch the locations from (e.g. a
bootstrap server, ...)