Fully support FatTuple and adaptive version storage #131
Description
Enhancement
Currently, only ChainedTuple is supported, which stores old versions to separate B+ trees, one for removal, and one for update.
For heavily updated tuples, storing old versions in separated B+ trees causes extra overhead on retrieving versions in removal or update B+ trees when getting visible tuples for update/lookup transactions or MVCC GC.
FatTuple stores all versions on the same page, which trades off space for performance. Based on the workload, we can adaptively transform the version storage for a tuple between off-row (ChainedTuple) and in-row (FatTuple).