consider domain support (optional?)

[tj]

seems like we can't really wrap just one stream so we may want to consider this, knocks a good 1000rps off but eh. seems a little tricky to do with middleware since we have to terminate it lower, similar issue to the compress middleware

[jonathanong]

i don't see the point of domains with koa since now i get ridiculously long error traces that now give me too much information. plus, right now i find it annoying since it catches things like syntax errors or references errors and it takes me a little longer to debug vs. just crashing.

either way, it should be optional and separate from core.