core: Decommissioning a node

[jseldess]

Roadmap: https://airtable.com/tblD3oZPLJgGhCmch/viwdV1sM5KEK2L14K/recGdsZPsKLSD26fY
RFC: https://github.com/cockroachdb/cockroach/blob/master/docs/RFCS/decommission.md
Issues:

• decommissioning: iterate over cli commands cockroach#17419
• cli: add command or flag to actively remove node from cluster cockroach#6198

PRs:

• Decommissioning a node (i) cockroach#16528
• server: hook up and test node decommissioning cockroach#17272

Test:

• https://github.com/cockroachdb/cockroach/blob/master/pkg/acceptance/decommission_test.go

[jseldess]

Although AirTable shows this as in "development", with cockroachdb/cockroach#17272, it seems like this is ready to be documented for 1.1, so I've applied the ready label. @tschottdorf, please chime in if you disagree.

[tbg]

Ready-ish. @m-schneider is revamping the cli commands: cockroachdb/cockroach#17419

But we should be able to write the docs soon.

[tbg]

I'm done here, @jseldess. Assigning you because I don't think there's anything actionable left on my end.