Improve async backend and bring initial reactive support (neo4j#999)

* Add support for Resolver and DomainNameResolver to async Testkit backend (neo4j#992)

* Decouple TestkitState from CommandProcessor. (neo4j#993)

This commit remoces the command processor from the TestKit state. The processor is supposed to use that state, not to be part of it.
However, some requests, like `NewDriver` need the processor to trigger further state.
Therefor the processor is now provided as injectable value via Jackson, so that any request can indicate that it needs a processor via a constructor argument.

* Remove unnessary Lombok noise. (neo4j#995)

* Migrate Testkit backend to Netty implementation (neo4j#994)

* Migrate Testkit backend to Netty implementation

* Remove unnessary Optional in processAsync.

This brings both sync and asynchronous paths in TestKitState in symmetry.
In addition, I noticed that the transaction related tests behaved differently in sync and async (when a tx is not found). I fixed that.

* Remove redundant lines

Co-authored-by: Michael Simons <[email protected]>

* Add reactive backend support (neo4j#998)

This update brings reactive backend support.

Its scope is limited to providing partial transparent support for existing test cases. More updates are expected in future PRs.

Co-authored-by: Michael Simons <[email protected]>

Author: injectives