PR to port TO server check utilities from perl to go

[Jason Tucker <ja...@gmail.com>]

For consideration, I have just submitted the following PR:

https://github.com/apache/trafficcontrol/pull/4010

This ports (most of) the TO extension servercheck tools from perl to go:

- 10G (IPv4 ping)
- 10G6 (IPv6 ping)
- ILO (ILO ping)
- MTU (MTU ping)
- DSCP (DSCP validator, many logic fixes, add SSL support)
- CDU (Cache Disk Usage)
- CHR (Cache Hit Ratio)
- FQDN (DNS check, now with PTR support)

The main impetus for this was the DSCP checker - I found that the perl
version, along with logic problems and lack of SSL support, has very
serious performance problems for a large-scale environment. The legacy
check was taking >20 hours to scan our environment (while skipping all
HTTPS delivery services!). The go version now includes HTTPS services, and
cut the total scan time roughly in half.

We've been using these new go versions in production now for a few months.

Thanks,

__Jason