Add flat_static_buffer::reset

include/boost/beast/core/impl/flat_static_buffer.ipp

@@ -44,6 +44,14 @@ mutable_data() ->
     return {in_, dist(in_, out_)};
 }
 
+inline
+void
+flat_static_buffer_base::
+reset()
+{
+    reset_impl();
+}
+
 inline
 auto
 flat_static_buffer_base::
@@ -61,6 +69,16 @@ reset(void* p, std::size_t n)
     reset_impl(p, n);
 }
 
+template<class>
+void
+flat_static_buffer_base::
+reset_impl()
+{
+    in_ = begin_;
+    out_ = begin_;
+    last_ = begin_;
+}
+
 template<class>
 void
 flat_static_buffer_base::