Upgradable staking pool contract

[bowenwang1996]

The current staking pool contract is not upgradable, which causes a lot of trouble when we need to change it for some reason. For example, if we were to introduce a new staking action, the current staking pool would no longer be compatible.

Some considerations here:

• Documentation on upgrading contracts https://github.com/near/near-sdk-rs/blob/master/HELP.md#upgrading-a-contract.
• It is not sufficient to make it upgradable. We also need to make sure that the upgrade process is safe and secure. The contract is supposed to be permissionless, i.e, the contract will not have any access key. Therefore it is important to figure out how to prevent owner of the contract from deploying a malicious contract that can transfer out all the tokens. One idea is to have a whitelist (stored in some whitelist contract) and the staking pool contract would read the whitelist first and check whether the hash of the new contract is whitelisted.

[janewang]

Based on updated Q4 OKR, this issue is removed from NodeX team
cc @bowenwang1996 @posvyatokum