Add timelocks to migrations and upgrades

[srdtrk]

Description

Support for contract upgradability by a proxy admin was added in #188 and support for light client migrations was added in #196. In both cases, we have not included any timelocks on said upgrades/migrations.

A timelock is a delay between the admin broadcasting a proposal and the execution of that proposal, allowing users to see the upcoming changes and potentially migrate their funds if they believe their funds to be at risk.

Discussion

Openzeppelin has a very useful TimelockController contract which is intended to be owned by a multisig.

This gives us two options:

1. Document that we recommend admins to use TimelockController (in this case we close this issue as not planned)
2. Deploy TimelockController for the admins ourselves and require them to use it.

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For Admin Use

• Not duplicate issue
• Appropriate labels applied
• Appropriate contributors tagged/assigned