util: make chunkbuffer non-quadratic on Windows The old str-based += collector performed very nicely on Linux, but turns out to be quadratically expensive on Windows, causing chunkbuffer to dominate in profiles. This list-based version has been measured to significantly improve performance with large chunks on Windows, with negligible overall overhead on Linux (though microbenchmarks show it to be about 50% slower). This may increase memory overhead where += didn't behave quadratically. If we want to gather up 1G of data to join, we temporarily have 1G in our list and 1G in our string.