Add drain_buffer class

fix #457

doc/3_3_buffers.qbk

@@ -47,6 +47,15 @@ To suit various needs, several implementation of dynamic buffer are available:
     output areas equal to the size of the underlying mutable buffer sequence.
     The implementation does not perform heap allocations.
 ]]
+[[
+    [link beast.ref.drain_buffer `drain_buffer`]
+][
+    A drain buffer has a small internal buffer and maximum size that
+    uses no dynamic allocation. It always has a size of zero, and
+    silently discards its input. This buffer may be passed to functions
+    which store data in a dynamic buffer when the caller wishes to
+    efficiently discard the data.
+]]
 [[
     [link beast.ref.flat_buffer `flat_buffer`]
     [link beast.ref.basic_flat_buffer `basic_flat_buffer`]

doc/quickref.xml

@@ -165,6 +165,7 @@
             <member><link linkend="beast.ref.buffered_read_stream">buffered_read_stream</link></member>
             <member><link linkend="beast.ref.buffers_adapter">buffers_adapter</link></member>
             <member><link linkend="beast.ref.consuming_buffers">consuming_buffers</link></member>
+            <member><link linkend="beast.ref.drain_buffer">drain_buffer</link></member>
             <member><link linkend="beast.ref.error_category">error_category</link></member>
             <member><link linkend="beast.ref.error_code">error_code</link></member>
             <member><link linkend="beast.ref.error_condition">error_condition</link></member>