Make speed control command friendly

## Feature Request

### Describe your feature request related problem:

User want to increase the balance speed, but now user need to turn a number for some limit:
```
region-schedule-limit
replica-schedule-limit
store-balance-rate
store limit 100 region-add
store limit all 100 region-remove
max-pending-peer-count
max-snapshot-count
```
Too many settings.
### Describe the feature you'd like:

consider adding some commands to adjust the speed in `pd-ctl`, such as:
```
store limit set scene idle
store limit set scene busy
store limit set scene default
```

Start at the pd-ctl entry point and trace the existing `store limit` command before deciding how the proposed scene commands fit. Confirm what idle, busy, and default should change among the listed speed settings, then verify each command produces the intended balance-speed behavior.

Written by the indexing model from the issue text.