Add per-node maintenance mode

[dotnwat]

The core primitive of rolling upgrade is preparing a node for being shutdown gracefully. This is primarily achieved by reliably relinquishing leadership of raft groups. A per-node maintenance mode should be created that achieves this state.

Fist new leadership should be rejected:

• raft: configurable rejection of raft leadership #3016

Next, leadership should be transfer to other nodes. This might require throttling in some cases:

• raft: bound leadership transfer time with blocking/throttling #3017

A node should be able to be put into maintenance mode:

• via admin api
• via signal (subject to any configuration that controls signal handling behavior)

A node should be able to be brought out of maintenance mode. If triggered via signal (e.g. shutting down as part of a k8s replica set rolling upgrade) this may not be feasible.

Additional notes

• RFC https://docs.google.com/document/d/1FcHekvM62RYRZqZaIvHbKucIQSGuSU2uuP6RW5r-oko/edit#
• PRD https://docs.google.com/document/d/140T_4Rg9cfIaTuPNlsEW-nfZ3INiLBHUxFy4-HsB3CY/edit#

[jcsp]

I was opening tickets for this before it was cool :) #2093 (but this one has more useful context, so closing the old one)

[dotnwat]

Duplicate of #3705