Feature: In-Memory (non-database, non-sql) DataManagers

**Is your feature request related to a problem? Please describe.**

Running hundreds of BPMN processes (sort of as 'lambda functions') per second against in-memory databases (H2 or HSQLDB) run into performance and locking problems.

**Describe the solution you'd like**

A faster, less database heavy option to execute BPMN processes, where the entities are not stored to SQL at all, but instead are kept in-memory in a (Concurrent) Map.

**Additional context**

The implementation does not need to provide full transaction support, as the intention is to run (mostly synchronous, though asynchronous must also work) short lived BPMN processes on it. If they fail in any non-expected way, the implementation is free to simply discard data (though creating a DeadLetter job would be good) while making sure the entities created within the failed process are also removed upon a critical failure.