Locking simplification

[drinkcoffee]

The locking implementation as currently written allows for multiple calls to the same contract, if the calls are within the same crosschain call. In some rare situations when a read or write is executed out of natural execution order relative to a write, an attacker could create a malicious invalid update.

The protocol has been revised to prevent this attack by simplifying the locking mechanism: any updates prevent any further updates to the contract. The lockable storage contract needs to be updated to reflect this update in the protocol.

[drinkcoffee]

Resolved with #5