Pre-Vote相关问题

在PreVote的逻辑中，节点使用current_term+1发起投票，其它的节点判断如果PreVote的request.LogId >= this.LogId就投票。考虑这样的场景：
3个节点ABC，A leader, B和C是follower。某个时间段，C与AB网络隔离，C发起PreVote，当ABC网络恢复时，A和B会投票给C，那么C就会elect_self，进而成为主。

请参考[修复PreVote流程的疑似bug](https://github.com/sofastack/sofa-jraft/issues/15#issuecomment-471527807)

Start by reading the linked SOFA JRaft discussion and tracing braft's PreVote request handling and vote-granting logic. Reproduce the three-node A/B/C partition and recovery scenario described here, then verify that a previously isolated follower cannot become leader solely from a PreVote using the newer term; add or update a regression test if the relevant test location is found.

Written by the indexing model from the issue text.