detect: delay tx cleanup in some edge case

detect: delay tx cleanup in some edge case

Ticket: 7552

f->sgh_toserver may be NULL but because FLOW_SGH_TOSERVER is unset
and thus, we want to delay cleanup until detection has really been
run with the right signature group head.

This may happen for a rule using
`alert tcp any any -> any any` and
a app-layer keyword to client
with a app-layer supporting both udp and tcp
with stream.midstream=true
and with the first packet of a flow being a server response

In this case, we swap the flow and reset its signature group heads