Use MongoDB Reactive Streams internally to provide a sync and an async API

Is your feature request related to a problem? Please describe.
I really want to use Morphia because of the clean API that the default MongoDB Java Driver does not provide. There is the "new" "POJO" feature, but you can't for instance update POJOs without writing manual queries. But Morphia does not support the "new" MongoDB Reactive Streams Java Driver, which can be used both for non-blocking and blocking requests.

Describe the solution you'd like
It would be great if the MongoDB Reactive Streams Java Driver could be implemented internally. Afterwards, an asynchronous Morphia API could be added in addition to the synchronous one. This could be realized by just using different Subscribers.

Describe alternatives you've considered
One alternative would be to use Morphia and deal with Threads by oneself but as I saw in other issues (#1289 and #829) I think I'm not the only one who would like an approach like this.

Additional context
None

Start by reviewing Morphia's existing synchronous API and the MongoDB Reactive Streams Java Driver, including the related issues #1289 and #829. Done would require an internal reactive implementation plus both synchronous and asynchronous Morphia APIs, but this issue names no files or tests.