Add pause option to on_headers interface:

* on_headers returns enum body_what
* body_what::pause lets caller receive headers during the parse

doc/quickref.xml

@@ -68,6 +68,7 @@
           </simplelist>
           <bridgehead renderas="sect3">Constants</bridgehead>
           <simplelist type="vert" columns="1">
+            <member><link linkend="beast.ref.http__body_what">body_what</link></member>
             <member><link linkend="beast.ref.http__connection">connection</link></member>
           </simplelist>
           <bridgehead renderas="sect3">Concepts</bridgehead>