Add support for asynchronous invocation entry

[sczyh30]

Describe what this PR does / why we need it

Add support for asynchronous invocation entry so that users can use Sentinel in asynchronous logic.

Does this pull request fix one issue?

Resolves #31

Describe how you did it

Internal

• Add several asyncEntry method in Sph interface and SphU class.
• Add a new AsyncEntry class for asynchronous entry representation, which resembles the linked CtEntry. The difference:
  • When calling SphU.asyncEntry, the created async entry is set to current context. As soon as the entry passed the slot chain (or blocked), it will be cleared from current context intermediately so that it won't affect the proceeding entries.
  • When the created async entry successfully passed the slot chain in CtSph#asyncEntryInternal, the async context inside the async entry will be initialized. We can get the async context via AsyncEntry#getAsyncContext method and use with ContextUtil#runOnContext to do context switching in asynchronous callback.
• Add runOnContext and replaceContext method in ContextUtil for context switching.

For users

We can call SphU.asyncEntry(xxx) to entry for asynchronous invocation, then exit it in the callback (or other place, often in different thread). If we want nested entry in asynchronous entry, we have to wrap the logic within the async context via ContextUtil.runOnContext(asyncContext, lambda).

Here is an example:

public void testAsync() {
    try {
        final AsyncEntry entry = SphU.asyncEntry(resourceName);

        CompletableFuture.runAsync(() -> {
            ContextUtil.runOnContext(entry.getAsyncContext(), () -> {
                try {
                    TimeUnit.SECONDS.sleep(2);
                    
                    // Call another sync resource under async context.
                    doAnotherSync();
                } catch (InterruptedException e) {
                    // Ignore.
                } finally {
                    // User should exit the resource in the callback.
                    entry.exit();
                }
            });
        });
    } catch (BlockException ex) {
        // Resource blocked. 
        ex.printStackTrace();
    }
}

Describe how to verify it

See test cases.

[CLAassistant]

All committers have signed the CLA.

[codecov-io]

Codecov Report

Merging #146 into master will increase coverage by 2.06%.
The diff coverage is 79.31%.

@@            Coverage Diff             @@
##             master   #146      +/-   ##
==========================================
+ Coverage     45.94%    48%   +2.06%     
- Complexity      563    639      +76     
==========================================
  Files           115    120       +5     
  Lines          3844   4081     +237     
  Branches        536    577      +41     
==========================================
+ Hits           1766   1959     +193     
- Misses         1862   1886      +24     
- Partials        216    236      +20

Impacted Files	Coverage Δ	Complexity Δ
...java/com/alibaba/csp/sentinel/context/Context.java	69.69% <100%> (+12.55%)	15 <3> (+5)	⬆️
.../com/alibaba/csp/sentinel/context/ContextUtil.java	78.72% <100%> (+5.03%)	11 <3> (+3)	⬆️
...e/src/main/java/com/alibaba/csp/sentinel/SphU.java	80% <33.33%> (-11.67%)	12 <1> (+1)
.../src/main/java/com/alibaba/csp/sentinel/CtSph.java	54.54% <50%> (-11.37%)	12 <2> (+4)
...main/java/com/alibaba/csp/sentinel/AsyncEntry.java	88% <88%> (ø)	8 <8> (?)
...rc/main/java/com/alibaba/csp/sentinel/CtEntry.java	94.28% <94.28%> (ø)	13 <13> (?)
...ntinel/slots/statistic/metric/WindowLeapArray.java	68% <0%> (-12%)	5% <0%> (-1%)
...a/csp/sentinel/slots/statistic/base/LeapArray.java	73.17% <0%> (-3.76%)	24% <0%> (+9%)
...a/csp/sentinel/slots/DefaultSlotsChainBuilder.java	100% <0%> (ø)	2% <0%> (?)
... and 12 more

---

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[CarpenterLee]

LGTM

[wavesZh]

as you said:

When calling SphU.asyncEntry, the created async entry is set to current context. As soon as the entry passed the slot chain (or blocked), it will be cleared from current context intermediately so that it won't affect the proceeding entries.

Other processing will get wrong entry for the context during SphU.asyncEntry changed curEntry, it's ok?