bypass: add explicit flag in stream engine

bypass: add explicit flag in stream engine

TCP reassembly is now deactivated more frequently and triggering a
bypass on it is resulting in missing some alerts due forgetting
about packet based signature.

So this patch is introducing a dedicated flag that can be set in
the app layer and transmitted in the streaming to trigger bypass.

It is currently used by the SSL app layer to trigger bypass when
the stream becomes encrypted.