Count object extension headers in bytes, not fields (#723)

This makes it possible to skip those without parsing, and easier to
parse without trial decoding.

Fixes #718

Author: ianswett

draft-ietf-moq-transport.md

@@ -2344,8 +2344,8 @@ Track or Datagram.
 * Object Status: As enumeration used to indicate missing
 objects or mark the end of a group or track. See {{object-status}} below.
 
-* Object Extension Count: The number of Object Extensions present. A value of 0
-  indicates that no Object Extension Headers are present.
+* Object Extension Length: The total length of the Object Extension Headers
+  block, in bytes.
 
 * Object Extensions : A sequence of Object Extension Headers. See
   {{object-extensions}} below.
@@ -2449,7 +2449,7 @@ OBJECT_DATAGRAM {
   Group ID (i),
   Object ID (i),
   Publisher Priority (8),
-  Extension Count (i),
+  Extension Headers Length (i),
   [Extension headers (...)],
   Object Payload Length (i),
   [Object Status (i)],
@@ -2517,7 +2517,7 @@ The Object Status field is only sent if the Object Payload Length is zero.
 ~~~
 {
   Object ID (i),
-  Extension Count (i),
+  Extension Headers Length (i),
   [Extension headers (...)],
   Object Payload Length (i),
   [Object Status (i)],
@@ -2621,7 +2621,7 @@ Each object sent on a fetch stream after the FETCH_HEADER has the following form
   Subgroup ID (i),
   Object ID (i),
   Publisher Priority (8),
-  Extension Count (i),
+  Extension Headers Length (i),
   [Extension headers (...)],
   Object Payload Length (i),
   [Object Status (i)],
@@ -2674,7 +2674,7 @@ STREAM_HEADER_GROUP {
 }
 {
   Object ID = 0
-  Extension Count  = 2
+  Extension Headers Length = 33
     { Type = 4
       Value = 2186796243
     },