nghttp2_sesson_mem_recv: Process incoming data in streaming fashion

Now incoming data is processed in very small buffer (up to 8 bytes)
using state machine. GOAWAY debug data can get to 16K - 1, and we
don't have callback for it. Since we don't want to buffer that
amount of data just for debugging, we currently discard it.
This change also makes parse_error callback not function.
It probably be removed from API.

lib/nghttp2_int.h

@@ -45,7 +45,9 @@ typedef int (*nghttp2_compar)(const void *lhs, const void *rhs);
    inclusive. */
 typedef enum {
   NGHTTP2_ERR_CREDENTIAL_PENDING = -101,
-  NGHTTP2_ERR_BUFFER_ERROR = - 102
+  NGHTTP2_ERR_BUFFER_ERROR = -102,
+  NGHTTP2_ERR_IGN_HEADER_BLOCK = -103,
+  NGHTTP2_ERR_IGN_PAYLOAD = -104,
 } nghttp2_internal_error;
 
 #endif /* NGHTTP2_INT_H */