[FLOG] Improve Log library about final_libs HOT 9 CLOSED

Additional, add a stop interface may better than now, we can stop it first before fork.

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Stop thread of log system has some risk, so we can use sync-write mode before fork. :D

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For sync-write mode, we'll add a new interface flog_set_mode(int mode);

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Ok, here is the final changes list:

1. Add new interface flog_set_mode(int mode). -- we can use it to control logging behavior, sync or async.
2. Adjust thread noticing mechanism, using "eventfd" and "epoll" to finish push and pop action. -- That means, we can fetch data from queue of thread simply.
3. Add a timer to control when we'll flush the cache to disk.

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Finish first version of new log system of using thread caching technology. Expect it'll more powerful. More testing will be continue.
On the other hand, I've fix some bugs of other modules during log system adjustment, such as "fev", "fhash", "fmempool".

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Most features has been completed, there are some action items next to do:

1. Add log_set_buffer interface -- user will use this interface to custom their buffer size per thread
2. Add log_register_event_xx interfaces -- when there are some event occur, log system will callback user function to do some notice or cleanup

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Above two feature has been checked-in.

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Need another important event -- LOG_EVENT_USER_BUFFER_RELEASED

By this event, we can monitor log system easily and quit safety. :D

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All done.

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