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/websocket/utf8_checker.cpp

@@ -379,7 +379,7 @@ public:
                 consuming_buffers<
                     boost::asio::const_buffers_1> cb{
                         boost::asio::const_buffers_1(s.data(), n)};
-                multi_buffer b{size};
+                multi_buffer b;
                 while(n)
                 {
                     auto const amount = (std::min)(n, size);