Updating improvements

[nkryuchkov]

Did you run make format && make check? Yes

Fixes #372, fixes #350, fixes skycoin/skybian#31

After merging, https://github.com/SkycoinProject/skywire-mainnet/releases/tag/v0.2.4-test-b7cfc168 should be removed.

Changes:

• Specify URL to download from
• Specify stable/beta target
• Update hypervisor itself
• Accept both --config and no --config for skywire-visor config (otherwise, updating from older versions would be broken)
• Fix buildinfo path
• Update VPN apps
• Fix updating if a release contains new apps
• Fix updating algorithm when supervised by systemd
• Skip updating binary if it is not found in an update

How to test this PR:

1. Checkout skywire-services against feature/holepunch-server branch
2. Run generic integration env
3. Send different curl requests based on this one:

curl -k -d '{"checksums_url":"", "archive_url":"", "channel":"", "version":""}' -X POST https://localhost:8000/api/visors/024ec47420176680816e0406250e7156465e4531f5b26057c9f6297bb0303558c7/update

Possible values:

• empty/no values do not modify the behavior; all empty values set default updater behavior
• channel sets updating channel; stable is the latest release, testing is the latest pre-release; stable is default
• version is a git version; overrides channel
• checksums_url and archive_url set links for downloading directly; they override version and channel

4. Send these curl requests:

curl -k -X GET https://localhost:8000/api/visors/024ec47420176680816e0406250e7156465e4531f5b26057c9f6297bb0303558c7/update/available
curl -k -X GET https://localhost:8000/api/visors/024ec47420176680816e0406250e7156465e4531f5b26057c9f6297bb0303558c7/update/available/stable
curl -k -X GET https://localhost:8000/api/visors/024ec47420176680816e0406250e7156465e4531f5b26057c9f6297bb0303558c7/update/available/testing

[Darkren]

Great job!

[evanlinjin]

Code looks good. I'll give it a test.