Optimize creation and use of a barrier_bucket.
Optimize creation and use of a barrier_bucket.

* buckets/barrier_buckets.c
  (struct barrier_context_t): This structure with only one pointer is not
        needed, we can store the pointer value directly in the data member
        of serf_bucket_t.
  (serf_bucket_barrier_create,
   serf_barrier_read,
   serf_barrier_read_iovec,
   serf_barrier_readline,
   serf_barrier_peek,
   serf_barrier_destroy,
   serf_barrier_get_remaining,
   serf_barrier_set_config): Update to not use barrier_context_t.

zhengcq315/serf,KyleSanderson/serf,zhengcq315/serf,alex1818/serf,x-clone/serf,KyleSanderson/serf,zhengcq315/serf,yesoce/serf,KyleSanderson/serf,SirJson/serf,google-code-export/serf,x-clone/serf,soufianeEL/serf,zhengcq315/serf,google-code-export/serf,alex1818/serf,soufianeEL/serf,x-clone/serf,lazysoez/serf,alex1818/serf,lazysoez/serf,SirJson/serf,stoneCC/serf,lazysoez/serf,zhupan/serf,SirJson/serf,pengdu/serf,yesoce/serf,SirJson/serf,lazysoez/serf,yesoce/serf,yesoce/serf,pengdu/serf,google-code-export/serf,alex1818/serf,stoneCC/serf,zhupan/serf,KyleSanderson/serf,zhupan/serf,soufianeEL/serf,zhupan/serf,pengdu/serf,x-clone/serf,pengdu/serf,stoneCC/serf,stoneCC/serf,google-code-export/serf,soufianeEL/serf