fix(txn): discard empty transactions on CommitWith (#2031)

## Problems
* Transactions with empty `pendingWrites` were never discarded(and
marked as done) for `CommitWith`
* This is similar to #2018,
which solved the same problem for `Commit`, but not for `CommitWith`
* The `CommitWith` is used by `WriteBatch`, so flushing an empty batch
never discarded the inner shadowed transaction, causing a multitude of
issues.

## Solution
Make sure `Discard` is called for `CommitWith` when `pendingWrites` are
empty. The existing unit test is updated to assert that.

batch_test.go

@@ -103,7 +103,12 @@ func TestEmptyWriteBatch(t *testing.T) {
 			wb = db.NewWriteBatch()
 			require.NoError(t, wb.Flush())
 			wb = db.NewWriteBatch()
+			// Flush commits inner txn and sets a new one instead.
+			// Thus we need to save it to check if it was discarded.
+			txn := wb.txn
 			require.NoError(t, wb.Flush())
+			// check that flushed txn was discarded and marked as read.
+			require.True(t, txn.discarded)
 		})
 	})
 	t.Run("managed mode", func(t *testing.T) {

txn.go

@@ -718,6 +718,8 @@ func (txn *Txn) CommitWith(cb func(error)) {
 		// callback might be acquiring the same locks. Instead run the callback
 		// from another goroutine.
 		go runTxnCallback(&txnCb{user: cb, err: nil})
+		// Discard the transaction so that the read is marked done.
+		txn.Discard()
 		return
 	}