Standardize the naming of the scheduling operator

## Enhancement Task

As of https://github.com/tikv/pd/commit/4f5a78a9014a5001844facaabf2586b99aa2ed44, we have a total of 48 scheduling operators in PD, and the namings of them are kind of arbitrary. I think we should propose a convention to standardize the naming of the scheduling operator.

For example, it could have triple parts like this:

```
{source}-{operation}-{type}
```

Take `make-up-replica` as an example, a better naming may be like:

```
replica_checker-add_peer-make_up_replica
```

It could tell us where the operator comes from and what it will do.