This PR fixes multiple issues with the UDP connection pre-negotiation feature:
- the enable condition was wrong (it checked the existence of the DiscV5 struct where it wasn't initialized yet, disabling the feature even if discv5 was enabled)
- the server pool queried already connected nodes when the discovery iterators returned them again
- servers responded positively before they were synced and really willing to accept connections
Metrics are also added on the server side that count the positive and negative replies to served connection queries.
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
Felföldi Zsolt