Fix conflict detection for managed DB

[roysc]

I propose this simple fix for detecting conflicts in managed mode. Addresses https://discuss.dgraph.io/t/fatal-error-when-writing-conflicting-keys-in-managed-mode/14784.

When a write conflict exists for a managed DB, an internal assert can fail:

badger/txn.go

Line 616 in aaab253

y.AssertTrue(commitTs != 0)

This occurs because a detected conflict is indicated with commitTs of 0, but handling the error is skipped for managed DB instances:

badger/txn.go

Line 563 in aaab253

if commitTs == 0 && !txn.db.opt.managedTxns {

.

Rather than conflate conflict detection with a timestamp of 0, it can be indicated with another return value from hasConflict.

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[NamanJain8]

Thanks @roysc, for the fix. This looks good to me.