This commit introduces upgradeability of the `RewardsStreamerMP`
contract by leveraging the UUPS pattern.
This means, for deployment, we have to first deploy an instance of
`RewardsStreamerMP` contract as a "template" logic contract and then
create a ERC1967Proxy that points to it.
The proxy ensures the implementation address is stored in a
deterministic storage slot.
This will later be leveraged by the `StakeVault` contract to implement
the functionality to leave the system in case there was a malicious
upgrade.
Closes#22
This adds a new emergency mode that can be enabled by the owner of the system.
When in emergency mode, stakers or `StakeVault`s can leave the system immediately.
This also applies when there was a malicious upgrade and a call to
`emergencyModeEnabled()` panics.
To have this in a fully secure manner, we still have to add the counter
part of "leaving" the system. This will allow users that don't agree
with a (malicious) upgrade to get their funds out of the vaults
regardless.
Closes#66
