同步过程
选取从哪个节点同步后，拉取oplog
1.Applies the op执行这个op日志
2.Writes the op to its own oplog (also local.oplog.rs)将这个op日志写入到自己的oplog中
3.Requests the next op请求下一个op日志secondary节点同步到哪了
secondary节点同步到哪了
主节点根据从节点获取oplog的时间戳可以判断数据同步到哪了
How does primary know where secondary is synced to? Well, secondary is querying primary‘s oplog for more results. So, if secondary requests an op written at 3pm, primary knows seconday has replicated all ops written before 3pm.
So, it goes like:
1.Do a write on primary.
2.Write is written to the oplog on primary, with a field “ts” saying the write occurred at time t.
3.{getLastError:1,w:2} is called on primary. primary has done the write, so it is just waiting for one more server to get the write (w:2).
4.secondary queries the oplog on primary and gets the op
5.secondary applies the op from time t
6.secondary requests ops with {ts:{$gt:t}} from primary‘s oplog
7.primary updates that secondary has applied up to t because it is requesting ops > t.
8.getLastError notices that primary and secondary both have the write, so w:2 is satisfied and it returns.
同步原理
如果A从B同步数据，B从C同步，C怎么知道A同步到哪了?看oplog读取协议：
当A从B同步数据，B对A说，我从年度oplog同步数据，如果你有写操作，告诉我一下。
B回答，我不是主节点，等我转发一下；B就跟主节点C说，就当做我是A，我代表A从你这同步数据。这时B与主节点C有两个连接，一个是B自己的，一个是代表A的。
A向B请求ops(写操作)，B就转向C，这样来完成A的请求。
A B C
<====>
<====> <—->
<====> 是”真正”的同步连接. <—-> “ghost” connection，B代表A与C的连接。
初始化同步
新增成员或者重做同步的时候，会进行初始化同步。
如下7步：
1.Check the oplog. If it is not empty, this node does not initial sync, it just starts syncing normally. If the oplog is empty, then initial sync is necessary, continue to step #2:检查oplog,如果空的，需要进行初始化同步，否则进行普通的同步。
2.Get the latest oplog time from the source member: call this time start.取同步来源节点最新的oplog time，标记为start
3.Clone all of the data from the source member to the destination member.复制所有数据到目标节点
4.Build indexes on destination. 目标节点建索引，2.0版本包含在复制数据步骤里，2.2在复制数据后建索引。
5.Get the latest oplog time from the sync target, which is called minValid.取目标节点最新的oplog time,标记为minValid
6.Apply the sync target’s oplog from start to minValid.在目标节点执行start 到minValid之间的oplog
7.Become a “normal” member (transition into secondary state).成为正常的成员
个人理解，start 到minValid之间的oplog是复制过来的没有执行的oplog，没有完成最终一致性的那部分，就是一个oplog replay的过程。
查看源码rs_initialsync.cpp,同步初始化步骤如下：
/**
clone data 复制数据的过程：
