Plugin pooling / cancellation

[Ben Wiley]

A big issue is that when jobs start to pile up we run into problems with stability. This is more an issue when longer plugins are being processed.

We probably want to:

1. Return a cancellation function from any request to perform a sample transform, which will kill the iframe but won't raise an error
2. Probably three instances of a plugin at any given time.
3. By default when using a plugin we will check if we need to kill the previous run, and then try to use an empty plugin instance, or wait for one.
4. At idle time we can create whatever instances we need to.à
5. test all of this in prod