feat: memtable bitmap for null

[ShiKaiWi]

Rationale

For a table which contains many nullable columns, many nulls may cause huge memory consumption in the memtable, leading to small ssts where null values will be compressed.

Detailed Changes

• Introduce a bitset for continuous row encoding
• Use u32 to encode the offest in the encoded bytes rather than u64

Test Plan

Existing unit test. And test it with tsbs's data, here is result with write buffer 32MB:

description	version	encoded size of one row	number of rows in memtable	sst's size
20 col (not null)	main	325B	70000+	1.8M
20 col (not null)	current	295B	78000	2M
20 col + 200 null col	main	2135B	12100	460K
20 col + 200 null col	current	332B	70000	1.5M

[jiacai2050]

LGTM