Add cron contract

[ethanfrey]

This is inspired by https://github.com/confio/priv_wasmd/pull/3 but using the new APIs, especially SubMsg.

The cron job will register as a BeginBlock Handler. One blocks with registered jobs, it will execute them (with a gas-limited sandbox) and store the result (ok or error) for future queries. Jobs that error will not automatically be re-run.

When registering a job, a user must pay upfront for the gas that will be used in the native fee token. This is converted to a gas allowance using the consensus minimum gas at the point of registration.

Requires confio/tgrade#7 for min gas

Update: more and more of our contracts want to do some delayed action. We should get a basic version of this working (with a whitelist of contracts that can set these jobs), so we don't keep adding the same logic everywhere.

We can add the more complex public gas-charging one later. See #113 and #136

[alpe]

I assume we want the cron contract on start up. An accept list for the contracts adds some complexity into the bootstrapping. If we want a "light" version for our contracts then we could decouple this with a new privilege so that we can use the sudo callback here for registration and also receiving triggers. I would expect this to become a Go module for better UX.

A public available version would be more complex with tokenomics and CLI support. The contract registry may be handy here as well

The original spike was using begin_block. I think an end_block may be more effective if we want to act on the new data.

[ethanfrey]

After reading this, I realise it is easier for privileged contracts to just implement end_block handlers and not try to centralise this. A cron job makes much more sense for permission less contracts who must pay gas and have some limits.

I will remove this from 0.5 milestone and deal with it later as a "value added" contract

[ueco-jb]

@ethanfrey is this one still valid? From your previous response it seems like not.

[ethanfrey]

It is still a valid wish.
It is more a design question about letting people pre-pay for gas.

But I think it will be a very useful feature.
Not ready to implement without some more prep work though

[ethanfrey]

Great idea, but for another repo