Include contract staking votes in candidates storage

### What would you like to be added

We can include contract staking votes in candiate storage, instead of calculating every time

### Why is this needed

more efficient

### How important you think this is for IoTeX
- [] must have
- [] should have
- [x] nice to have

### Additional information
Let us know any background or context that would help us better understand the request (for example the particular use-case that prompted this request)