p2p/enode: use unix timestamp as base ENR sequence number (#19903)

This PR ensures that wiping all data associated with a node (apart from its nodekey)
will not generate already used sequence number for the ENRs, since all remote nodes
would reject them until they out-number the previously published largest one.

The big complication with this scheme is that every local update to the ENR can
potentially bump the sequence number by one. In order to ensure that local updates
do not outrun the clock, the sequence number is a millisecond-precision timestamp,
and updates are throttled to occur at most once per millisecond.

Co-authored-by: Felix Lange <fjl@twurst.com>