fix(Txn): serializable behavior is broken #2057

[al8n]

Hi! This PR is a try to fix #2057.

For the details, please see #2057 (comment)

This PR changes conflictKeys field from conflictKeys map[uint64]struct{} to conflictKeys map[uint64]int. The int is the length of the reads []uint64; store it so that when checking indirect conflict, we know the new write for each key happens after how many reads. And finally, when check conflicts, introduce a new loop to check logical conflicts (indirect conflicts).

After the changes, badger's transaction model can handle this kind of write skew https://wiki.postgresql.org/wiki/SSI#Intersecting_Data.

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[al8n]

Closed as this solution cannot handle

db: [a1, a2, b1, b2]
txn1: read [a1, a2] -> sum range "a".."b" -> insert b3
txn2: read [b1, b2] -> sum range "b".."c" -> insert a3
txn2: commit
txn1: commit (fail) logical dependent