This PR implements the first one of the "lespay" UDP queries which
is already useful in itself: the capacity query. The server pool is making
use of this query by doing a cheap UDP query to determine whether it is
worth starting the more expensive TCP connection process.
* les: move serverPool to les/vflux/client
* les: add metrics
* les: moved ValueTracker inside ServerPool
* les: protect against node registration before server pool is started
* les/vflux/client: fixed tests
* les: make peer registration safe
