Improve docs and add error log related to illegal activation of transaction

Using multiple nested transactions, ElasticApm.currentTransaction() returns oldest transaction instead of newest created.

Activating a transaction transaction.activate() pushes the transaction to the head of the active stack: ElasticApmTracer.java:

    public void activate(TraceContextHolder<?> holder) {
        if (logger.isDebugEnabled()) {
            logger.debug("Activating {} on thread {}", holder, Thread.currentThread().getId());
        }

        ((Deque)this.activeStack.get()).push(holder);
    }

But when retrieving it, the last element of the stack is checked: ElasticApmTracer.java:

  public Transaction currentTransaction() {
        TraceContextHolder<?> bottomOfStack = (TraceContextHolder)((Deque)this.activeStack.get()).peekLast();
        if (bottomOfStack instanceof Transaction) {
            return (Transaction)bottomOfStack;
       ....................

In the above method, peekLast() returns the oldest transaction instead of newly pushed one.

As multiple methods use Deque::peek(first element) to get active transaction, the logic inside currentTransaction() method should be change to iterate from first to last element:

     @Nullable
     public Transaction currentTransaction() {
-        TraceContextHolder<?> bottomOfStack = (TraceContextHolder)((Deque)this.activeStack.get()).peekLast();
+        TraceContextHolder<?> bottomOfStack = (TraceContextHolder)((Deque)this.activeStack.get()).peek();
         if (bottomOfStack instanceof Transaction) {
             return (Transaction)bottomOfStack;
         } else {
-            Iterator it = ((Deque)this.activeStack.get()).descendingIterator();
+            Iterator it = ((Deque)this.activeStack.get()).iterator();

             TraceContextHolder context;
             do {

elastic-apm-agent-1.9.0