Support dropping non-Raft nodes

[otoolep]

With this change dropping non-Raft nodes is supported.

[otoolep]

This is still work-in-progress, and is mostly code from @corylanou

The idea here is that we get incremental support out for this command, for very specific situations.

cc @jwilder

[otoolep]

This doesn't compile, but it's the check I want to add. Once this check is in place, we can forget about all the Raft complexity until later.

[otoolep]

Started with #4233.

This a minimum viable fix for the "extra" nodes reporting in #4212.

[otoolep]

I'd like feedback from @corylanou and @jwilder

[otoolep]

Raft-check updated to take place at a higher-level, code now compiles.

[otoolep]

Tested:

> show servers
id      cluster_addr    raft    raft-leader
1       localhost:8088  true    true
2       localhost:8188  true    false
3       localhost:8288  true    false

> drop server 3
ERR: node is a Raft node
Warning: It is possible this error is due to not setting a database.
Please set a database with the command "use <database>".
> 

[otoolep]

Patch ready for merging, 4 green CI builds.

https://circleci.com/gh/influxdb/influxdb/tree/drop_node_non_raft

[corylanou]

Could use ErrNodeRaft here to lessen the brittleness of this test.

[corylanou]

This looks good. Limiting it to non-raft nodes takes out most of the real hard problems and allows us to move forward in another PR to do it right.

+1

[toddboom]

👍 for getting this out as an intermediate step.

[otoolep]

Made the change suggested by @corylanou -- merging on green.