1.
In the previous implementation, the max thread num of olap scanner was set relatively small, such as 3.
which would slow down some of queries.
In this PR, I changed the max thread num to a quarter of the scaner thread pool(default is 12),
which is less than the old scan node's max thread num, but larger than the previous implementation.
The upper limit of the max thread num of the old scan node is too high, which is not reasonable.
2.
Lower down the number of pre allocated free blocks.
2fb218173e