TS-2156: Fix stats trap in different type of threads

There are two ways to create threads in ATS:
1) By EventProcessor::spawn_event_threads(), such as ET_NET threads.
2) By EventProcessor::spawn_thread(), such as flush thread.

But I found that we can't stats the same things across these two type of
threads.

This patch will make stats in both type of threads transparent for upper
layer developers.

BTW: renames "dthreads" to "all_dthreads" to keep naming consistency with
     "all_ethreads".

Signed-off-by: Yunkai Zhang <qiushu.zyk@taobao.com>