improve ContextManager.stopSpan performance: call ThreadLocal only once (#3068)

* improve ContextManager.stopSpan and awaitFinishAsync performance: call ThreadLocal only once
For the [JMH benchmark test](https://gitlab.com/qxo1/Common-JMH-benchmark/-/jobs/251025992) :
On gitlab CI(java 8): call ThreadLocal once vs twice fast over 10%
My T480 java 8 : fast over 40%; java 11: fast over 50%;

[The JMH benchmark code ](https://github.com/qxo/Common-JMH-benchmark/blob/master/src/main/java/test/ThreadLocalGetXTimeBenchmark.java)

* Update apm-sniffer/apm-agent-core/src/main/java/org/apache/skywalking/apm/agent/core/context/ContextManager.java
Co-Authored-By: Nkezhenxu94 <kezhenxu94@163.com>

* Update apm-sniffer/apm-agent-core/src/main/java/org/apache/skywalking/apm/agent/core/context/ContextManager.java
Co-Authored-By: Nkezhenxu94 <kezhenxu94@163.com>

* Update apm-sniffer/apm-agent-core/src/main/java/org/apache/skywalking/apm/agent/core/context/ContextManager.java
Co-Authored-By: 吴晟 Wu Sheng <wu.sheng@foxmail.com>

apm-sniffer/apm-agent-core/src/main/java/org/apache/skywalking/apm/agent/core/context/ContextManager.java

@@ -150,11 +150,12 @@ public class ContextManager implements BootService {
     }
 
     public static AbstractTracerContext awaitFinishAsync(AbstractSpan span) {
-        AbstractSpan activeSpan = activeSpan();
+        final AbstractTracerContext context = get();
+        AbstractSpan activeSpan = context.activeSpan();
         if (span != activeSpan) {
             throw new RuntimeException("Span is not the active in current context.");
         }
-        return get().awaitFinishAsync();
+        return context.awaitFinishAsync();
     }
 
     /**
@@ -165,12 +166,22 @@ public class ContextManager implements BootService {
         return get().activeSpan();
     }
 
+    /**
+    * Recommend use ContextManager::stopSpan(AbstractSpan span), because in that way, 
+    * the TracingContext core could verify this span is the active one, in order to avoid stop unexpected span.
+    * If the current span is hard to get or only could get by low-performance way, this stop way is still acceptable.
+    */
     public static void stopSpan() {
-        stopSpan(activeSpan());
+        final AbstractTracerContext context = get();
+        stopSpan(context.activeSpan(),context);
     }
 
     public static void stopSpan(AbstractSpan span) {
-        if (get().stopSpan(span)) {
+        stopSpan(span, get());
+    }
+
+    private static void stopSpan(AbstractSpan span, final AbstractTracerContext context) {
+        if (context.stopSpan(span)) {
             CONTEXT.remove();
             RUNTIME_CONTEXT.remove();
         }