refactor: consolidate database pools (#9105)

# Which Problems Are Solved

Zitadel currently uses 3 database pool, 1 for queries, 1 for pushing
events and 1 for scheduled projection updates. This defeats the purpose
of a connection pool which already handles multiple connections.

During load tests we found that the current structure of connection
pools consumes a lot of database resources. The resource usage dropped
after we reduced the amount of database pools to 1 because existing
connections can be used more efficiently.

# How the Problems Are Solved

Removed logic to handle multiple connection pools and use a single one.

# Additional Changes

none

# Additional Context

part of https://github.com/zitadel/zitadel/issues/8352

internal/eventstore/v3/push.go

@@ -4,9 +4,11 @@ import (
 	"context"
 	"database/sql"
 	_ "embed"
+	"fmt"
 
 	"github.com/zitadel/logging"
 
+	"github.com/zitadel/zitadel/internal/api/authz"
 	"github.com/zitadel/zitadel/internal/database"
 	"github.com/zitadel/zitadel/internal/eventstore"
 	"github.com/zitadel/zitadel/internal/telemetry/tracing"
@@ -55,6 +57,11 @@ func (es *Eventstore) writeCommands(ctx context.Context, client database.Context
 		}()
 	}
 
+	_, err = tx.ExecContext(ctx, fmt.Sprintf("SET LOCAL application_name = '%s'", fmt.Sprintf("zitadel_es_pusher_%s", authz.GetInstance(ctx).InstanceID())))
+	if err != nil {
+		return nil, err
+	}
+
 	events, err := writeEvents(ctx, tx, commands)
 	if err != nil {
 		return nil, err