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.
[Bug][BetaRowset] fix beta rowset read slowly with limit
beta rowset do not update raw_rows_read in statistics and will read all
data in tablet when query with limit, which lead to long query time.
