Footer 存储了index/meta_index的offset，还存了magic标识
Metaindex 存储了 [meta block: filter] .、[meta block: properties] [meta block: compression dictionary]、[meta block: range deletion tombstone]、 [metaindex block] 的 offset
那么这些block上都存了什么 ？比如：
Filter: filter_policy
properties : column_family_id， column_family_name compression :compression library's dictionary