Speeds up WriteUtf8 significantly when dealing with strings made by the concatenation of many others.
This is to allow eventual realloc without messing up the data_ references in all the slices.
However it's not working very well: Hitting a 'hello world' server with many requests (ab -t 60 -c 10) will cause it to crash with the following error. Obtained 3 stack frames. ./node(_Z11print_tracev+0x1c) [0x80d1b3c] ./node(_ZN4node6Parser7ExecuteERKN2v89ArgumentsE+0x69) [0x80d3759] ./node [0x811f44b] TypeError: Already parsing a buffer at Socket.<anonymous> (/home/ryan/projects/node/lib/http2.js:393:20) at IOWatcher.callback (/home/ryan/projects/node/lib/net.js:81:12) at node.js:985:9 at node.js:989:1