Add Meteor._wrapAsync with couple tests
Use a noop callback if client code doesn't provide one
Add tests for async server-side mongo writes.
Rewrite wrapAsync, with numRequiredArgs argument.

See comments for examples of how to use it.
Use wrapAsync in some call sites
WIP: Run collection writes asynchronously on server.

The basic idea is that LocalCollection and _Mongo objects now have write methods
that take optional callbacks, and they run asynchronously and call the callback
when done. (For LocalCollection, nothing is actually asynchronous but just
appears to be.) The write methods on Collection wrap these write methods using
Meteor._wrapAsync.

Tests are passing, but needs to be cleaned up.
Make numRequiredArguments optional.

Adds too much complexity for not very much benefit.