Simplify txn commits and aborts (#2491)

Txn:

- Commits have been separated into CommitToMemory and CommitToDisk. This logic is a lot simpler and clearer than before. In particular, CommitToDisk does not try to generate any state. It just serializes the posting list stored at startTs and pushes that to Badger.
- Multiple txns doing CommitToDisk can now be batched up into fewer transactions and use callbacks to execute quickly. Added a helper struct for that.
- Remove the concept of activeTxns in posting/list.go. It's a repetition.
- Simplify txn.deltas, and removed as much as I could.
- Removed abortMutation methods from txn (mvcc.go) and posting/list.go

Zero:

- TryAbort returns OracleDelta, which can be directly proposed in the group.
- abortOldTransactions uses this to ensure that in case we missed a transaction update, the result of the abort can then be proposed to the group, so group snapshot doesn't get stuck.