Add async support

[edeandrea]

Add async support for document conversion

[edeandrea]

For this I'm thinking we add methods to the Api that return CompletableFuture and in the implementations we can do the task submission/polling/etc, ensuring we move off to worker threads.

[ThomasVitale]

I like the idea!

[edeandrea]

This depends on #188

[edeandrea]

@oscerd Would you want to take what you did with Camel and bring it here?

[edeandrea]

@all-contributors add @oscerd for design promotion

[allcontributors]

@edeandrea

I've put up a pull request to add @oscerd! 🎉

[edeandrea]

@all-contributors add @oscerd for ideas

[allcontributors]

@edeandrea

I've put up a pull request to add @oscerd! 🎉

[edeandrea]

@all-contributors add @jmb-streamsets for ideas design