Follower 节点日志和快照被清空后，仅接收心跳不会触发 snapshot 拉取

[houlin2016]

你好，我在使用 Braft 时遇到了一个问题，情况如下：

我们有一个三节点集群，节点 A、B、C。其中，B 节点的 raftlog 和 snapshot 数据被清空后重启。重启后，B 成功加入集群，成为 follower，并能收到 leader 的心跳。但在不写入新日志的情况下（即日志没有新增 index），B 节点不会触发 snapshot 拉取，始终卡在 (index=0, term=45)，状态如下所示：

B 节点状态（被清空后重启）：

state: FOLLOWER
readonly: 0
term: 45
conf_index: 0
last_msg_to_now: 95
election_timer: timeout(1000ms) SCHEDULING(in 214ms)
vote_timer: timeout(2000ms) STOPPED
stepdown_timer: timeout(1000ms) STOPPED
snapshot_timer: timeout(3600000ms) SCHEDULING(in 2161540ms)
storage: [1, 0]
disk_index: 0
known_applied_index: 0
last_log_id: (index=0,term=0)
state_machine: Idle
last_committed_index: 0
last_snapshot_index: 0
last_snapshot_term: 0
snapshot_status: IDLE

Leader 节点状态：

state: LEADER
readonly: 0
term: 45
conf_index: 1338
changing_conf: NO    stage: STAGE_NONE
election_timer: timeout(1000ms) STOPPED
vote_timer: timeout(2000ms) STOPPED
stepdown_timer: timeout(1000ms) SCHEDULING(in 37ms)
snapshot_timer: timeout(3600000ms) SCHEDULING(in 574758ms)
storage: [1348, 1347]
disk_index: 1347
known_applied_index: 1347
last_log_id: (index=1347,term=45)
state_machine: Idle
last_committed_index: 1347
last_snapshot_index: 1347
last_snapshot_term: 45
snapshot_status: IDLE
replicator_12408160521560@A: next_index=1348  flying_append_entries_size=0 idle hc=5804502 ac=19 ic=0
replicator_42408507084623@B: next_index=1348  flying_append_entries_size=0 idle hc=5804995 ac=1085 ic=1

[aschenmaker]

same as #340. & #292