Validator RPC and utils

[ilblackdragon]

We want to have validator tools available for all users: shell, explorer, applications.

Specifically:

• RPC to fetch validator data (done)
• Computing seat price (done)
• Computing validator schedule for any epoch (requires reimplemented random shuffle from nearcore)
• Given current validators and proposals, perspective next epoch (done)

See these comments for more details:
and near/nearcore#2259 (comment) and this PR for code:
near/near-cli#328 (comment)

[frol]

This issue has been mostly fulfilled. Reimplementing the validator schedule in JS is quite an effort, so I suggest we close this issue and either open a new one for the remaining item or only get back to it once we have a real need for that (it is advanced tooling for the validators, but currently we have a stable set of validators, so I don't think it is that important now).