router: investigate performance of SCMP parsing in router

[matzf]

Follow up on #4344

Since #4344, delivering an SCMP error message to an end host in the destination AS (the destination of the SCMP message, that is, i.e. the source AS of the offending packet) requires the router to parse the SCMP message. It inspects the quoted offending packet in the SCMP error message to determine the port from which the offending packet was sent. This is then used as the destination port for the SCMP error message.

The implementation of this procedure in the router is somewhat simplistic. It uses the gopacket.NewPacket API, which eagerly parses the full structure and allocates various internal helper objects. Instead, the router should likely only perform minimal, using DecodeFromBytes with re-usable layer objects.
Investigate how big the impact is of this eager parsing and temporary objects.