Dropped messages of AsyncLogWriter cause memleak

The obvious problem is the following code. msg contains a new string from os::strdup(msg). when we decide to drop the message, we need to call os::free for the string, or memory leaks.

void AsyncLogWriter::enqueue_locked(const AsyncLogMessage& msg) {
  if (_buffer.size() >= _buffer_max_size) {
    bool p_created;
    uint32_t* counter = _stats.add_if_absent(msg.output(), 0, &p_created);
    *counter = *counter + 1;

    // drop the enqueueing message.
    return;
  }


There's another not so obvious memleak issue. In AsyncLogMapIterator, we clear counters to zero. If the LogOutput has deleted by LogConfiguration::delete_output(), we should delete the corresponding entry of AsyncLogWriter::_stats. It's because user can delete/add logOutput instances via jcmd. hotspot gradually leaks memory if we don't drop deleted LogOutputs.