#4995
**Implementation of Separated Page Cache**
- Add config "index_page_cache_ratio" to set the ratio of capacity of index page cache
- Change the member of StoragePageCache to maintain two type of cache
- Change the interface of StoragePageCache for selecting type of cache
- Change the usage of page cache in read_and_decompress_page in page_io.cpp
- add page type as argument
- check if current page type is available in StoragePageCache (cover the situation of ratio == 0 or 1)
- Add type as argument in superior call of read_and_decompress_page
- Change Unit Test
1. BlockManager has been added into StorageEngine.
So StorageEngine should be initialized when starting BetaRowset unit test.
2. Cache should not use the same buf to store value, otherwise the address
will be freed twice and crash.
