JoinPointImpl: Remove thread-locals after usage, where possible

Avoid potential ThreadLocalMap.Entry accumulation. Entry is a
WeakReference. When JoinPointImpl objects are collected by GC, Entry
instances are still be referenced by ThreadLocalMap, which leads to
memory pressure and potentially more full GCs. So, we proactively remove
ThreadLocal<Integer> arcIndex instances when arcIndex has been
decremented back to -1 per thread. This is not perfect, because not each
thread can be expected to proceed, but it should ameliorate the
situation to some degree.

Fixes #302.

Co-authored-by: Alexander Kriegisch <Alexander@Kriegisch.name>
Signed-off-by: KimmingLau <294001791@qq.com>
Signed-off-by: Alexander Kriegisch <Alexander@Kriegisch.name>

runtime/src/main/java/org/aspectj/runtime/reflect/JoinPointImpl.java

 			arcs = new ArrayList<>();
 		}
 		if (arc == null) {
-			arcIndex.set(arcIndex.get() - 1);
+			int newIndex = arcIndex.get() - 1;
+			if (newIndex > -1)
+				arcIndex.set(newIndex);
+			else
+				arcIndex.remove();
 		}
 		else {
 			this.arcs.add(arc);