Selected DMSG Server

[mrpalide]

Did you run make format && make check? Yes

Fixes #1387

Changes:

• new flag for skywire-visor as --dmsg-server <pk> for set specific dmsg server during running visor.
• pass this flag to visor.New() and set in context in initDmsg for dmsg.Server() part

How to test this PR:

• checkout dmsg to Selected DMSG Server dmsg#199
• on go.mod comment out replace part for dmsg
• run make dep then make build
• from dmsg discovery all lists (prod or test, based on your visor config) choosing one dmsg server public key
• run visor by that key as --dmsg-server flag like ./skywire-visor -c skywire-config.json --dmsg-server 02113579604c79b704e169a4fd94fd78167b86fe40da1016f8146935babcc9abcb
• check logs of visor

[0pcom]

This is ok for our immediate purposes, but I would not expect such a setting to survive a config reload.

Even in the context of config gen / update, we don't have a way to really persist this setting either but we could use a flag for it. So please do come back with another PR which adds a flag for config gen, and for config update if you want.

What we really need is this

• a locally running conf service endpoint
• support via scripts or envs in the packaging for switching to use that by default
• set a different dmsg discovery in your local conf service endpoint and pull the rest from conf.skywire.skycoin.com
• your local dmsg discovery always shows the dmsg server you set first

this fits into the larger context of both user deployments of dmsg and robustly persisting settings across updates.

I won't complicate our testing efforts in the immediate term with this, but it is something for us to work toward