Opt performance of async log

[chenBright]

What problem does this PR solve?

Issue Number:

Problem Summary:

 thread_num=1 log_type=sync log_size=100 count=462130 duration=2s qps=231065 average_time=4349.05us
 thread_num=8 log_type=sync log_size=100 count=271989 duration=2s qps=135994 average_time=59007.1us
 thread_num=1 log_type=async log_size=100 count=1449393 duration=2s qps=724696 average_time=1383.17us
 thread_num=8 log_type=async log_size=100 count=952119 duration=2s qps=476060 average_time=17001.6us

目前异步日志（完全由异步线程落盘日志）的性能仅仅是同步日志的3倍左右，没达到每秒百万的级别。

通过cpu profiler分析发现，瓶颈在PrintLogPrefix，特别是localtime_r或者localtime_s（有锁）。

What is changed and the side effects?

Changed:

将PrintLogPrefix逻辑后移到异步线程运行，加快LOG(xxx)的速度。

优化后，异步日志性能得到了很大的提升，性能是同步日志的十几倍以上。

 thread_num=1 log_type=sync log_size=100 count=612247 duration=2s qps=306123 average_time=3287.74us
 thread_num=8 log_type=sync log_size=100 count=453624 duration=2s qps=226812 average_time=35352.2us
 thread_num=1 log_type=async log_size=100 count=9856504 duration=2s qps=4928252 average_time=203.911us
 thread_num=8 log_type=async log_size=100 count=14547148 duration=2s qps=7273574 average_time=1104.67us

Side effects:

• Performance effects(性能影响):

• Breaking backward compatibility(向后兼容性):

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Check List:

• Please make sure your changes are compilable(请确保你的更改可以通过编译).
• When providing us with a new feature, it is best to add related tests(如果你向我们增加一个新的功能, 请添加相关测试).
• Please follow Contributor Covenant Code of Conduct.(请遵循贡献者准则).

[wwbmmm]

LGTM

[Superskyyy]

Hi, let's merge this?

[chenBright]

@wwbmmm 这个PR应该可以合了

[wasphin]

这里对日志时间的影响是可接收的吗？

[chenBright]

日志时间应该是跟原来一样的，只影响落盘时间。

brpc/src/butil/logging.cc

Lines 1308 to 1314 in bdc141c

	info.severity = severity;
	info.timestamp = GetTimestamp();
	info.file = file;
	info.func = func;
	info.line = line;
	info.content = content.as_string();
	info.raw = true;