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/ostream.cpp

@@ -40,7 +40,7 @@ public:
                 "0123456789abcdef" "0123456789abcdef" "0123456789abcdef" "0123456789abcdef" 
                 "0123456789abcdef" "0123456789abcdef" "0123456789abcdef" "0123456789abcdef" 
                 "0123456789abcdef" "0123456789abcdef" "0123456789abcdef" "0123456789abcdef";
-            multi_buffer b(512);
+            multi_buffer b;
             ostream(b) << s;
             BEAST_EXPECT(boost::lexical_cast<std::string>(
                 buffers(b.data())) == s);