les: UDP pre-negotiation of available server capacity (#22183)

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.
# Conflicts:
#	les/client.go
#	les/clientpool.go
#	les/clientpool_test.go
#	les/enr_entry.go
#	les/server.go
#	les/vflux/client/serverpool.go
#	les/vflux/client/serverpool_test.go
#	les/vflux/server/balance.go
#	les/vflux/server/balance_test.go
#	les/vflux/server/prioritypool.go
#	les/vflux/server/prioritypool_test.go
#	p2p/nodestate/nodestate.go