+str #24229 remove protobuf changes, which do not need to be made in this PR
docs
moved things
config object
subscription timeout confifmed working, also, attributes
document attributes for sub timeout
tests for the source also failing when it should
additional demand test
implemented protection from materializing "in cycles"; would be nice in
types but that breaks the niceness of use of the types
SinkRef/SourceRef...
cleanup
no idle timeout built in, can use the Timeout stages
more docs
simplest change to prevent exposing SinkRef => SourceRef => SinkRef cycle
Things to decide:
* is it ok to require using `getSource` / `getSink` as Java API, is there better naming?
* where should the constructors go? I'd say just in regular javadsl/scaladsl `Source`/ `Sink` objects
move constructors to {javadsl,scaladsl}.{Source,Sink} companion objects
Remove now useless "canMaterialize" field
Separate stage (implementation) from ref (wrapped actor ref) to make it clearer what is serialized
Clarify that partner refs are not optional in on-the-wire interfaces
minor cleanup in SourceRefStage
Renamed the stages but questionable if that really helps ;)
cleanups, better docs
cleanup, fix docs compilation
fix mima
got rid of Futures in the materialized values of stream refs
