To ease a future transition into joint quorums, this commit removes the
previous "ad-hoc" majority-based quorum and vote computations with that
introduced in the `raft/quorum` package.
More specifically, the progressTracker now uses a quorum.JointConfig for
which the "second" majority quorum is always empty; in this case the
quorum behaves like the one quorum.MajorityConfig that is actually
present. Or, more briefly, this change is a no-op, but it will take the
busywork out of actually starting to make use of joint quorums in the
future.
On a side node, I suspect that this might've fixed a bug regarding the
read index though I haven't been able to explicitly come up with a
counter-example. The problem was that the acks collected for the read
index weren't taking into account membership changes, so they'd run the
danger of using acks from nodes since removed to claim that a quorum of
acks had been received. There's a chance that there isn't a
counter-example (the only guarantee extracted from the "quorum" is that
there isn't another leader, but even if there's another leader all that
matters is that that leader doesn't have a divergent history from the
stale leader in the hypothetical counter-example), but either way there
is morally a bug here that is now fixed because VoteCommitted doesn't
care about votes from members that are not voters known to the currently
active configuration.
advance() should use rs.req.Entries[0].Data as the context instead of
req.Context for deletion. Since req.Context is never set, there won't be
any context being deleted from pendingReadIndex; results mem leak.
FIXES#7571
gavwu