Define the contents of QPACK Encoder and Decoder streams (#1467)

Fixes #1361

This removes the length prefixed instruction blocks on the encoder stream and replaces them with an unframed sequence of instructions.

draft-ietf-quic-qpack.md

@@ -406,19 +406,8 @@ to an existing entry in the static or the dynamic table or as a string literal.
 For entries which already exist in the dynamic table, the full entry can also be
 used by reference, creating a duplicate entry.
 
-Each set of encoder instructions is prefaced by its length, encoded as a
-variable length integer with an 8-bit prefix.  Instructions MUST NOT span more
-than one block.
-
-~~~~~~~~~~ drawing
-     0   1   2   3   4   5   6   7
-   +---+---+---+---+---+---+---+---+
-   |       Block Length (8+)       |
-   +-------------------------------+
-   |     Instruction Block (*)   ...
-   +-------------------------------+
-~~~~~~~~~~
-{: title="Encoder instruction block"}
+The contents of the encoder stream are an unframed sequence of the following
+instructions.
 
 ### Insert With Name Reference
 
@@ -525,6 +514,9 @@ the server informs the client about the processing of the client's header blocks
 and table updates, and the client informs the server about the processing of the
 server's header blocks and table updates.
 
+The contents of the decoder stream are an unframed sequence of the following
+instructions.
+
 ### Table State Synchronize
 
 The Table State Synchronize instruction begins with the '00' two-bit pattern.