MEDIUM: stream-int: simplify si_alloc_conn()

MEDIUM: stream-int: simplify si_alloc_conn()

Since we now always call this function with the reuse parameter cleared,
let's simplify the function's logic as it cannot return the existing
connection anymore. The savings on this inline function are appreciable
(240 bytes) :

$ size haproxy.old haproxy.new
   text    data     bss     dec     hex filename
1020383   40816   36928 1098127  10c18f haproxy.old
1020143   40816   36928 1097887  10c09f haproxy.new