raft: fix deadlock during PreVote migration process

[siddontang]

refer etcd-io/etcd#8525

[csmoe]

The pre_vote field of Raft is private:

pub struct Raft<T: Storage> {
    ...
    pub check_quorum: bool,
    pre_vote: bool,
    ...

but pre_vote was changed directly in etcd-io/etcd#8525 :

func newPreVoteMigrationCluster(t *testing.T) *network {
    ...
    n1.preVote = true
    ... 

I couldn't find any methods to change the value of pre_vote after a raft was instantiated, how can I reset the pre_vote field of a raft?

[siddontang]

Hi @csmoe

Seem you can make the field public like check_quorum does