Add `@InitiatedBy` to the java docs on the responder flow, this is
already shown in the kotlin version.
Add a note on overriding responders, instructing developers to still
include the `@InitiatedBy` annotation on the new responder even though
the configuration setup can make developers think that defining the
override will guarantee the initiator and responder will join up
correctly.
* first attempt at a flowManager
fix test breakages
add testing around registering subclasses
make flowManager a param of MockNode
extract interface
rename methods
more work around overriding flows
more test fixes
add sample project showing how to use flowOverrides
rebase
* make smallest possible changes to AttachmentSerializationTest and ReceiveAllFlowTests
* add some comments about how flow manager weights flows
* address review comments
add documentation
* address more review comments
