Versioned task executor & auto-upgrade of the executor

[lukewwww]

To constantly add more types of supported tasks, the task executor should be upgraded. To let all the node operators to upgrade their nodes manually is a tough task.

There will always be different versions of nodes running in the network.

1. Allow the nodes to upgrade the executor automatically. The node should check new versions of executor periodically when it is online.
2. The version of the executor should be reported to the blockchain, who should assign tasks according to the version.