wal heartbeat: allow overriding query with env variable (#1725)

To enable heartbeating, set these two environment vars:

```yaml
  PEERDB_WAL_HEARTBEAT_QUERY: ${PEERDB_WAL_HEARTBEAT_QUERY:-UPDATE _peerdb_heartbeat SET last_heartbeat = now() WHERE id = 1;}
  PEERDB_ENABLE_WAL_HEARTBEAT: ${PEERDB_ENABLE_WAL_HEARTBEAT:-true}
```

flow/activities/flowable.go

@@ -512,13 +512,7 @@ func (a *FlowableActivity) SendWALHeartbeat(ctx context.Context) error {
 		return err
 	}
 
-	command := `
-		BEGIN;
-		DROP AGGREGATE IF EXISTS PEERDB_EPHEMERAL_HEARTBEAT(float4);
-		CREATE AGGREGATE PEERDB_EPHEMERAL_HEARTBEAT(float4) (SFUNC = float4pl, STYPE = float4);
-		DROP AGGREGATE PEERDB_EPHEMERAL_HEARTBEAT(float4);
-		END;
-		`
+	command := peerdbenv.PeerDBWALHeartbeatQuery()
 	// run above command for each Postgres peer
 	for _, pgPeer := range pgPeers {
 		activity.RecordHeartbeat(ctx, pgPeer.Name)

flow/peerdbenv/config.go

@@ -106,6 +106,15 @@ func PeerDBEnableWALHeartbeat() bool {
 	return getEnvBool("PEERDB_ENABLE_WAL_HEARTBEAT", false)
 }
 
+// PEERDB_WAL_HEARTBEAT_QUERY
+func PeerDBWALHeartbeatQuery() string {
+	return GetEnvString("PEERDB_WAL_HEARTBEAT_QUERY", `BEGIN;
+DROP AGGREGATE IF EXISTS PEERDB_EPHEMERAL_HEARTBEAT(float4);
+CREATE AGGREGATE PEERDB_EPHEMERAL_HEARTBEAT(float4) (SFUNC = float4pl, STYPE = float4);
+DROP AGGREGATE PEERDB_EPHEMERAL_HEARTBEAT(float4);
+END;`)
+}
+
 // PEERDB_ENABLE_PARALLEL_SYNC_NORMALIZE
 func PeerDBEnableParallelSyncNormalize() bool {
 	return getEnvBool("PEERDB_ENABLE_PARALLEL_SYNC_NORMALIZE", false)