In short: it seems leader can treat observers as quorum members.
Steps to repro:
1. I have a following ensemble configuration:
# servers list
2. I'm bringing up servers 1,2,3 and it's enough for quorum (1 and 2).
3. I'm shutting down the one from the quorum who is the follower.
As I understand, expected result is that leader will start a new election
round so that to regain quorum.
But the real situation is that it just says goodbye to that follower, and is
still operable. (When I'm shutting down 3rd one -- observer -- leader starts
trying to regain a quorum).
Is this a bug, or a feature?