Import a new ThreadPool Framework

[chaoyli]

Now, Doris have multiple ThreadPool in different code path.
And all of them have a little difference.
So I import a new ThreadPool to unify all of these.
And also to further use.

[chaoyli]

This ThreadPool have three features.

1. Using a token mechanism to receive tasks.
   In Doris, tasks are submitted into ThreadPool without group.
   If a task in one group should be handled serially, it will stall all tasks in ThreadPool.
   For example, MemTableFlush tasks belong to different tablets can be flushed concurrently
   and tasks belongs one tablet should be flushed serially.
   So we need a token mechanism to divide tasks into groups.

2. Support set min_threads and max_threads in ThreadPool.
   Now, Doris will start all threads in ThreadPool. It is not efficiently.
   We should start threads as little as possible if have no tasks.
   At peek time, ThreadPool can expand threads up to max_threads.

3. Better trace to resource usage of any thread than C++ library.
   All threads in ThreadPool are in black box. Resource usage in this ThreadPool is not clear.
   New thread implementation should handle this question.

[chaoyli]

1. import scoped_refptr from gutil [gutil] import scoped_refptr smart pointer from KUDU #2899 [done]
2. import ConditionVariable from Kudu [util] Import util tools from KUDU #2905
3. import ThreadPool [util] Import ThreadPool and Thread from KUDU #2915