BUG/MINOR: http: Call stream_inc_be_http_req_ctr() only one time per request

BUG/MINOR: http: Call stream_inc_be_http_req_ctr() only one time per request

The function stream_inc_be_http_req_ctr() is called at the beginning of the
analysers AN_REQ_HTTP_PROCESS_FE/BE. It as an effect only on the backend. But we
must be careful to call it only once. If the processing of HTTP rules is
interrupted in the middle, when the analyser is resumed, we must not call it
again. Otherwise, the tracked counters of the backend are incremented several
times.

This bug was reported in github. See issue #74.

This fix should be backported as far as 1.6.