waiting for majority of responses from unique voters

[andrewhsu]

In joint consensus raft needs majority of both new and previous quorums
to make the decisions. Voting for new leader is one of the processes
that requires majority agreement from both quorums. In previous
implementation we waited for majority of the vote request responses.
This approach would make leader election much slower in situations where
quorums differ by one node.

Example:

current voters: [1,2,4], previous voters: [1,2,3]

In this scenario to elect leader it is enough to wait from responses
from node 1 and 2 as they form majority in both quorums. In previous
implementation we wait for at least (n/2)+1 responses. Where n is a
number of unique voter ids. In the example above n = len([1,2,3,4]) = 4. This way we had to wait for 3 replies while only 2 of the are enough
to elect new leader.

[andrewhsu]

/backport v21.11.x

[andrewhsu]

closing this issue because #4157 was merged to dev branch