[MINOR] http: don't wait for sending requests to the server

By default we automatically wait for enough data to fill large
packets if buf->to_forward is not null. This causes a problem
with POST/Expect requests which have a data size but no data
immediately available. Instead of causing noticeable delays on
such requests, simply add a flag to disable waiting when sending
requests.
diff --git a/include/types/buffers.h b/include/types/buffers.h
index 34189cd..ba7bcb8 100644
--- a/include/types/buffers.h
+++ b/include/types/buffers.h
@@ -113,6 +113,7 @@
 
 #define BF_DONT_READ     0x1000000  /* disable reading for now */
 #define BF_EXPECT_MORE   0x2000000  /* more data expected to be sent very soon (one-shoot) */
+#define BF_SEND_DONTWAIT 0x4000000  /* don't wait for sending data (one-shoot) */
 
 /* Use these masks to clear the flags before going back to lower layers */
 #define BF_CLEAR_READ     (~(BF_READ_NULL|BF_READ_PARTIAL|BF_READ_ERROR|BF_READ_ATTACHED))