Revise common header processing

Instead of emitting common headers on decoder side, encoder
now keeps track of removed common headers and encode it
as necessary.

lib/nghttp2_hd.h

@@ -47,6 +47,11 @@ typedef enum {
   NGHTTP2_HD_SIDE_SERVER = 1
 } nghttp2_hd_side;
 
+typedef enum {
+  NGHTTP2_HD_ROLE_DEFLATE,
+  NGHTTP2_HD_ROLE_INFLATE
+} nghttp2_hd_role;
+
 typedef enum {
   NGHTTP2_HD_FLAG_NONE = 0,
   /* Indicates name was dynamically allocated and must be freed */
@@ -92,6 +97,8 @@ typedef struct {
      further invocation of inflate/deflate will fail with
      NGHTTP2_ERR_HEADER_COMP. */
   uint8_t bad;
+  /* Role of this context; deflate or infalte */
+  nghttp2_hd_role role;
 } nghttp2_hd_context;
 
 /*