fix: Discard index and subscription implicit transactions (#1715)

## Relevant issue(s)

Resolves #1714

## Description

Discard index and subscription implicit transactions.

I'm not sure if this was causing any problems, but they should be
discarded.

db/collection_index.go

@@ -403,6 +403,8 @@ func (c *collection) GetIndexes(ctx context.Context) ([]client.IndexDescription,
 	if err != nil {
 		return nil, err
 	}
+	defer c.discardImplicitTxn(ctx, txn)
+
 	err = c.loadIndexes(ctx, txn)
 	if err != nil {
 		return nil, err

db/subscriptions.go

@@ -15,6 +15,7 @@ import (
 
 	"github.com/sourcenetwork/defradb/client"
 	"github.com/sourcenetwork/defradb/client/request"
+	"github.com/sourcenetwork/defradb/datastore"
 	"github.com/sourcenetwork/defradb/events"
 	"github.com/sourcenetwork/defradb/planner"
 )
@@ -58,25 +59,37 @@ func (db *db) handleSubscription(
 			continue
 		}
 
-		p := planner.New(ctx, db.WithTxn(txn), txn)
+		db.handleEvent(ctx, txn, pub, evt, r)
 
-		s := r.ToSelect(evt.DocKey, evt.Cid.String())
+		txn.Discard(ctx)
+	}
+}
 
-		result, err := p.RunSubscriptionRequest(ctx, s)
-		if err != nil {
-			pub.Publish(client.GQLResult{
-				Errors: []error{err},
-			})
-			continue
-		}
+func (db *db) handleEvent(
+	ctx context.Context,
+	txn datastore.Txn,
+	pub *events.Publisher[events.Update],
+	evt events.Update,
+	r *request.ObjectSubscription,
+) {
+	p := planner.New(ctx, db.WithTxn(txn), txn)
 
-		// Don't send anything back to the client if the request yields an empty dataset.
-		if len(result) == 0 {
-			continue
-		}
+	s := r.ToSelect(evt.DocKey, evt.Cid.String())
 
+	result, err := p.RunSubscriptionRequest(ctx, s)
+	if err != nil {
 		pub.Publish(client.GQLResult{
-			Data: result,
+			Errors: []error{err},
 		})
+		return
 	}
+
+	// Don't send anything back to the client if the request yields an empty dataset.
+	if len(result) == 0 {
+		return
+	}
+
+	pub.Publish(client.GQLResult{
+		Data: result,
+	})
 }