Remote service workers

First, some things I love about Deno Deploy for context:

- it implements the `FetchEvent` for handling incoming requests just like service workers do inside a web browser
- `setInterval` can be used to do some task periodically (e.g. check some API that doesn't support pushing updates)
- code running in different regions can communicate with one another via `BroadcastChannel`
- some KV store is on the roadmap (see #76) and [Cache API](https://developer.mozilla.org/en-US/docs/Web/API/Cache) is [planned](https://deno.com/deploy/docs/runtime-api/#future-support) (I could see supporting an `IndexedDB` implementation down the road too)

What I feel is missing and that I suspect will become more important as a KV store and other things becomes available:

- `install` event for setting things up prior to replacing a prior deployment
- `activate` event for cleaning up things from a prior deployment
- `periodicsync` event if that helps in better managing background tasks (using `setInterval` may mean a deployment is never truly idle enough to be replaced by a new deployment)
- programmatic removal of prior deployments

I want to be able to use Deno Deploy a lot like local service workers (and with the same API where possible) except ran in a trusted environment (not in a user agent where code and application data can be modified).

I think that implementing more of the service worker API and events should enable stopping tasks in a prior deployment like periodic syncs, transfering data and/or upgrading caches/data/etc., and transitioning traffic to a new deployment while removing/disabling the prior one (allowing traffic to old ones could be problematic if an install of a new version changes cache/data schemas, etc.).

i.e. I want to be able to manage the life cycle of my deployments more and treat them more like service workers so that I can better transition tasks and data from deployment to deployment.