flow: make TCP reuse handling in flow engine optional

flow: make TCP reuse handling in flow engine optional

In case of autofp (or more general, when flow and stream engine
run in different threads) the flow engine should not trigger a flow
reuse as this can lead to race conditions between the flow and the
stream engine.

In such cases, the flow engine can be far ahead of the stream engine
as packets are in a queue between the threads.

Observed:

Flow engine tags packet 10 as start of new flow. Flow is tagged as
'reused'.

Stream engine evaluates packet 5 which belongs to the old flow. It
rejects the flow as it's tagged 'reused'. Attaches packet 5 to the
new flow which is wrong.

Solution:

This patch connects the flow engines handling of reuse cases to
the runmode. It hooks into the RunmodeSetFlowStreamAsync() call to
notify the flow engine that it shouldn't handle the reuse.