MEDIUM: buffer: use b_alloc() to allocate and initialize a buffer

b_alloc() now allocates a buffer and initializes it to the size specified
in the pool minus the size of the struct buffer itself. This ensures that
callers do not need to care about buffer details anymore. Also this never
applies memory poisonning, which is slow and useless on buffers.
diff --git a/src/session.c b/src/session.c
index d57a2d5..05fd72e 100644
--- a/src/session.c
+++ b/src/session.c
@@ -477,18 +477,15 @@
 	if (unlikely((s->req = pool_alloc2(pool2_channel)) == NULL))
 		goto out_free_task; /* no memory */
 
-	if (unlikely((s->req->buf = pool_alloc2(pool2_buffer)) == NULL))
+	if (unlikely(b_alloc(&s->req->buf) == NULL))
 		goto out_free_req; /* no memory */
 
 	if (unlikely((s->rep = pool_alloc2(pool2_channel)) == NULL))
 		goto out_free_req_buf; /* no memory */
 
-	if (unlikely((s->rep->buf = pool_alloc2(pool2_buffer)) == NULL))
+	if (unlikely(b_alloc(&s->rep->buf) == NULL))
 		goto out_free_rep; /* no memory */
 
-	/* initialize the request buffer */
-	s->req->buf->size = global.tune.bufsize;
-	b_reset(s->req->buf);
 	channel_init(s->req);
 	s->req->prod = &s->si[0];
 	s->req->cons = &s->si[1];
@@ -504,9 +501,6 @@
 	s->req->wex = TICK_ETERNITY;
 	s->req->analyse_exp = TICK_ETERNITY;
 
-	/* initialize response buffer */
-	s->rep->buf->size = global.tune.bufsize;
-	b_reset(s->rep->buf);
 	channel_init(s->rep);
 	s->rep->prod = &s->si[1];
 	s->rep->cons = &s->si[0];