Contract time out

[xcolwell]

Contracts need to be closed as timely as possible to avoid "insufficient balance" issues. There are two areas where we can clean up contracts:

• A contract should be usable for up to 15 minutes since creation. A contract should always be associated with a creation time, and the client should stop using a contract if it is more than 15 minutes since creation time. The contract should be closed (not checkpointed) at this time.
• The API should support a check contract route, that returns the contract creation time and whether the remaining transfer bytes on the contract. The receiver should async check this route for each contract.
• The protocol should support threading a contract creation time for inline contracts.
• The protocol should support a close of a contract. The sender should send a contract close whenever it locally closes a contract, e.g. on timeout or when the sequence closes.