correct icmp timestamp (request) and nearby cosmetics

Addressing [self-note](https://github.com/Exa-Networks/exabgp/blob/0105c4defaa9bcbcae14f0c0fac014c9ea62b644/lib/exabgp/protocol/ip/icmp.py#L23) in lib/exabgp/protocol/ip/icmp.py line 23.

The correct name for type 13 is indeed 'timestamp' and not 
'timestamp request'. This is consistent with JunOS, as well as
with RFC1323 and http://www.iana.org/assignments/icmp-parameters

This is most likely due to the way the mechanism functions in
that the request involves the sender setting its own timestamp.