multi_buffer implementation change (API Change):

fix #440

WARNING

multi_buffer constructor now takes a maximum size instead of the
"allocation size". This is a breaking change. The allocation size
feature is removed. To update calling code, remove the allocation
size parameter from call sites.

* multi_buffer now uses a geometric growth algorithm for better performance.

test/core/buffers_adapter.cpp

@@ -161,7 +161,7 @@ public:
         }
         {
             using sb_type = beast::multi_buffer;
-            sb_type b(2);
+            sb_type b;
             b.prepare(3);
             buffers_adapter<
                 sb_type::mutable_buffers_type> ba(b.prepare(8));