Is it possible and safe to fully replace etcd by pd

## Feature Request

### Describe your feature request related problem

Our current design using both PD for TiDB and ETCD for other service discovery and we know that PD is also has an ETCD embedded. So if we can reduce totally ETCD, it would be great and save us a lot of cost for maintenance.

### Describe the feature you'd like

PD can be compatible with ETCD and using of PD as ETCD will not break anything of TiDB.

### Describe alternatives you've considered

Maintain both ETCD and PD as separated services.