Check if we have to send ENR according to enr-seq

[ElFantasma]

When we handle WhoAreYou messages we need to decide if we have to send our ENR. Currently we are always sending it. We don't need to send it if the enr-seq is up to date.
From the spec:

node A compares the enr-seq element of the WHOAREYOU challenge against its own node record sequence number. If the sequence number in the challenge is lower, it includes its record into the handshake message packet.

See TODO comment here