Node Tree Lack | 调用链 不完整

## Issue Description

*bug report*

### Describe what happened (or what feature you want)

* First, when I want to construct one invocation tree like following :


* then, there is in-complete tree i get by invoking DefaultNode.printDefaultNode();

* following is what i got:
```
Entry-entrance(t:0 pq:2.0 bq:0.0 tq:2.0 rt:3.5 1mp:2 1mb:0 1mt:2)
-nodeB(thread:0 pq:1.0 bq:0.0 tq:1.0 rt:0.0 1mp:1 1mb:0 1mt:1)
-nodeA(thread:0 pq:1.0 bq:0.0 tq:1.0 rt:7.0 1mp:1 1mb:0 1mt:1)
--nodeC(thread:0 pq:2.0 bq:0.0 tq:2.0 rt:0.0 1mp:2 1mb:0 1mt:2)
```

### Describe what you expected to happen

* I thing there must be something wrong with the design of NodeSelectSlot.
* There should be a complete invocation tree.

### How to reproduce it (as minimally and precisely as possible)

* following is my testing code:
```
@Test
public void invocationChain() {
Context context = ContextUtil.enter("entrance");

nodeA();
nodeB();

context.getEntranceNode().printDefaultNode();
context.exit();
}

private void nodeA() {
if (SphO.entry("nodeA")) {
try {
nodeC();
} finally {
SphO.exit();
}
}
}

private void nodeB() {
if (SphO.entry("nodeB")) {
try {
nodeC();
} finally {
SphO.exit();
}
}
}

private void nodeC() {
if (SphO.entry("nodeC")) {
try {
System.out.println("nodeC");
} finally {
SphO.exit();
}
}
}

static {
List list = Lists.newArrayList(Arrays.asList("nodeA", "nodeB", "nodeC"));
List rules = Lists.newArrayList();
for(String str : list) {
FlowRule rule1 = new FlowRule();
rule1.setResource(str);
rule1.setCount(2);
rule1.setGrade(RuleConstant.FLOW_GRADE_QPS);
rule1.setLimitApp("default");
rules.add(rule1);
}
FlowRuleManager.loadRules(rules);
}
```

### Tell us your environment

* java 1.8
* Sentinel 1.8.2

### Anything else we need to know?

* nothing

Start by reproducing the invocationChain test and inspect DefaultNode.printDefaultNode(), the ContextUtil.enter flow, and NodeSelectSlot behavior around SphO.entry and SphO.exit. Trace the node relationships for nodeA, nodeB, and nodeC, then add or update a regression test showing that the printed invocation tree is complete.

Written by the indexing model from the issue text.