Include contract staking votes in candidates storage

[envestcc]

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)