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>
TS-2145: Add metrics for log collation

Without accurate stats, we don't known whether the log is lost.
This patch is used to verify:

1) At log client side, how many logs genterated from HttpSM, how many
   logs sent to LogServer.

2) At log server side, how many logs received from LogClient, how many
   logs written to LogServers's disk.

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