* 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
