Change udp memory allocation scheme from uv_buf_init to slab allocation.

[mrb]

Takes slab allocation scheme from stream_wrap. Discussed with @ry and @bnoordhuis in IRC. This produced demonstrably better memory usage for a high volume of packets. This has been raised in #2468 amongst other issues. Please see this gist: https://gist.github.com/68890a9ae60e6b0f3bc0 for log output and test files.

[bnoordhuis]

Having stream_wrap.cc and udp_wrap.cc share code would be preferable. Can you move the static variables into a struct? That way, sharing slabs (or not) between streams and UDP is easy to tweak.

[mrb]

@bnoordhuis The only reason I kept them separate was because @ry recommended that as an easy first step, since it seems to be a point of contention what will happen with the udp_wrap code. Additionally I can see some other potential advantages of not tying the implementations to each other -- soon we'll have configurable slabs, etc., where a few lines of duplicated code isn't going to kill anyone.

[bnoordhuis]

Memory usage is indeed more stable and raw throughput doesn't seem to suffer. Thanks Michael, landed in bd9fa2e.

[mrb]

Thanks!