bugfix: The time difference between the raft node and the follower node cannot synchronize data.

[PeppaO]

• I have registered the PR changes.

Ⅰ. Describe what this PR did

在部署raft集群时，发现有的节点启动慢时间差会导致follwing同步失败，且initSync不会再次同步.
同步时机：

1. 定时器syncMetadata，第一次同步失败initSync设置为true后，会导致同步元数据不会再次发起
2. onFollowing同步，由于leader为null，同步元数据无法成功

以上导致该节点无法加入到raft集群中。
该pr主要是针对以上两个时机出现的异常情况进行设置，保证定时器syncMetadata还会进入同步逻辑。

Ⅱ. Does this pull request fix one issue?

Ⅲ. Why don't you add test cases (unit test/integration test)?

Ⅳ. Describe how to verify it

Ⅴ. Special notes for reviews

[codecov]

Codecov Report

Attention: Patch coverage is 20.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 51.79%. Comparing base (795f274) to head (a0cc162).
Report is 1 commits behind head on 2.x.

Files with missing lines	Patch %	Lines
...he/seata/server/cluster/raft/RaftStateMachine.java	20.00%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                2.x    #7150      +/-   ##
============================================
+ Coverage     51.78%   51.79%   +0.01%     
- Complexity     6755     6757       +2     
============================================
  Files          1151     1151              
  Lines         40970    40975       +5     
  Branches       4803     4803              
============================================
+ Hits          21215    21222       +7     
+ Misses        17760    17758       -2     
  Partials       1995     1995              

Files with missing lines	Coverage Δ
...he/seata/server/cluster/raft/RaftStateMachine.java	14.41% <20.00%> (+0.12%)	⬆️

... and 2 files with indirect coverage changes

[funky-eyes]

LGTM