Add 'applier' and 'inspector' commands to server

[timvaillancourt]

Description

This PR introduces two new commands for the unix-socket/TCP server:

1. applier - list the MySQL hostname of the applier
2. inspector - list the MySQL hostname of the inspector

In case this PR introduced Go code changes:

• contributed code is using same conventions as original code
• script/cibuild returns with no formatting errors, build errors or unit test errors.

cc @gtowey / @TeeKraken / @shlomi-noach

[shlomi-noach]

LGTM

[timvaillancourt]

Example output w/applier+inspector on same host:

tim@Tims-MacBook-Pro /tmp % echo applier | nc -U /tmp/gh-ost.test.test.sock
Host: 4031462df3a9:3306, Version: 5.7.33-log
tim@Tims-MacBook-Pro /tmp % echo inspector | nc -U /tmp/gh-ost.test.test.sock
Host: 4031462df3a9:3306, Version: 5.7.33-log

That's the final tweak before merging