[Proposal] Delete index storage structure design for batch delete

[ZhangYu0123]

Is your feature request related to a problem? Please describe.
The detail delete index design in storage level for batch delete function feature #4051 .
We discuss the concrete delete index structure and update the design document here.

Describe the solution you'd like
When importing and deleting according to the key, we will generate a bitmap index of the delete key for these keys. The content of the index is: the deleted line number (ordinal).
(1) The specific format of delete index footer page(DeleteIndexFooterPB):

             +--------------------+
             |        type        |
             |--------------------|
             |  uncompressed_size |
             |--------------------|
             |    content_bytes   |
             |--------------------|
             |      num_items     |
             |--------------------|
             |      checksum      |
             +--------------------+

-type: The type is DELETE_INDEX_PAGE, which means that the page of the index type is deleted
-uncompressed_size: Uncompressed page size
-content_bytes: Data size of index content
-num_items: Store the number of deleted entries recorded in the index
-checksum: Checksum of index page content

(2) SegmentFooterPB changes:
Add PagePointerPB delete_index_page to SegmentFooterPB, because delete index is Segment level index.

(3) PageTypePB changes:
Add DELETE_INDEX_PAGE to PageTypePB to describe the type of delete index.
Add DeleteIndexFooterPB to PageFooterPB as optional.

(4) encoding
The encoding format of index using roaring bitmap format.