Improve mirror selection process about cpcache HOT 2 OPEN

ICMP is not directly supported by Erlang and no hex.pm repository for ICMP exists, so testing the TCP connection delay would probably be more straightforward. Consider the delay for DNS resoution, which we don't want to measure. Something like:

:inet.getaddr('ipv4.xnet.space', :inet) # -> {:ok, {78, 46, 175, 29}}
:timer.tc(:gen_tcp, :connect, [{78, 46, 175, 29}, 80, [:binary]])

from cpcache.

Looks like the new experimental socket API does support ICMP:
http://erlang.org/doc/man/socket.html

from cpcache.