Some sctp tests based on existing tcp tests.

- test_conflate
- test_immediate_sctp
test_abstract_ipc is only relevant on linux avoid in cmake build.
Add optional overrides to stream_engine for os read and write functions.

Intention is to allow sctp implementation to provide its own read and
write functions.
This should probably done differently (ie with some kind of abstract interface)
Going for minimal change to experiment.