Move the logic to decide if we redispatch to a new server from
sess_update_st_cer() to a new inline function, stream_choose_redispatch(), and
use it in do_l7_retry() instead of just setting the state to SI_ST_REQ.
That way, when using L7 retries, we won't redispatch the request to another
server except if "option redispatch" is used.
This should be backported to 2.0.
4bd5867627