!str #15121 MaterializerSettings now uses a config object

* it's now easier to create a FlowMaterializer(), which will use the default (see akka-streams/.../reference.conf) configuration
* kept methods to easily overload some values of the settings (used to be done via copy())
* made some apply() duplication trade-offs in FlowMaterializer companion for nicer API
* made it possible to set namePrefix from Java API

Resolves #15121
Hm, I guess it should – thanks.
In `http` the mailbox that performs this check is disabled, so I didn't notice (tests green etc).