domain: move error handling directly into instance

[trevnorris]

Instead of doing all the domain handling in core, allow the domain to
set an error handler that'll take care of it all. This way the domain
error handling can be abstracted enough for any user to use it.

[trevnorris]

@isaacs just need a quick approval.

This is going to add some new functionality that we're going to pretend doesn't exist. For example, you can do the following:

process.domain = {
  _errorHandler: function errorHandler(er) {
    return true;
  },
  enter: function enter() { },
  exit: function exit() { }
}

setTimeout(function() {
  throw new Error('freak yeah!');
});

And... it'll catch all errors. Basically another uncaughtException.

This is a first step at working towards #6011, and since it can be done separate I'd like to get it in now to lighten the review load necessary from the other.

[bnoordhuis]

LGTM in the sense that I don't see anything obviously wrong. Isaac should probably sign off on it as well though.

[trevnorris]

I'm going to infer @isaacs is good with this. From IRC:

Aug 21 16:13:51 <trevnorris>  isaacs: do you mind a change like this: https://github.com/trevnorris/node/compare/domain-api-redo-again-sucka
Aug 21 16:14:16 <trevnorris>  isaacs: hm, here. this is w/o the multi-context stuff: https://github.com/trevnorris/node/commit/5764104
Aug 21 16:15:08 <isaacs>      trevnorris: that seems fine