feat: discard batch safely after commit batch

[rfyiamcool]

change

1. discard batch safely.
2. lock/unlock safely.

refer:

• https://pkg.go.dev/github.com/dgraph-io/badger#Txn.Discard
• https://pkg.go.dev/github.com/dgraph-io/badger#WriteBatch.Cancel

[rfyiamcool]

add ut for discard.

[roseduan]

I think this is a little complicated, like we have to maintain the locked field.

A better simple solution is to add a Rollback function:

• clear pendingWrites if any
• make rollbacked field as true
• unlock
• add check if rollbacked, just return an error

so the user will either call commit or rollback.

[rfyiamcool]

I think this is a little complicated, like we have to maintain the locked field.

A better simple solution is to add a Rollback function:

• clear pendingWrites if any
• make rollbacked field as true
• unlock
• add check if rollbacked, just return an error

so the user will either call commit or rollback.

done, review.

[roseduan]

I think the locked field can be removed?

[roseduan]

use will either call Commit or Rollback, so need to call this in defer

[rfyiamcool]

done