coderd: implement control loop to reconcile templates' prebuild state

[dannykopping]

Retrieve desired, actual, and extraneous values from #299 and initiate new provisioner jobs to result in the desired number of instances per template.

The reconciliation loop must occur periodically (configurable?), but must also be triggerable imperatively.

[dannykopping]

Status update:

#299 is now completed, I currently have a query in https://github.com/coder/coder/compare/dk/prebuilds called GetTemplatePrebuildState which fetches the values specified above for a given template. Additionally, it will now take in-progress builds into account, and I also took a basic stab at determining how many instances we need to create or delete in coder/coder@4ea5bbd.