The ManyToOneImplementationHolder stores an upgradeable implementation address in storage, which many-to-one proxies query at execution time to determine which contract to delegate to. The manager can upgrade the implementation address by calling the holder with the abi-encoded address as calldata. If any other account calls the implementation holder, it will return the implementation address. This pattern was inspired by the DharmaUpgradeBeacon from 0age https://github.com/dharma-eng/dharma-smart-wallet/blob/master/contracts/upgradeability/smart-wallet/DharmaUpgradeBeacon.sol
Fallback function for the contract. Used by proxies to read the implementation address and used by the proxy manager to set the implementation address. If called by the owner, reads the implementation address from calldata (must be abi-encoded) and stores it to the first slot. Otherwise, returns the stored implementation address.