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.
Remove numRequiredArgs argument.
Update comment
Fix args to LocalCollection callback.

Added tests for local collections with no connections, which is where this
problem was manifesting itself.
Make LocalCollection and _Mongo writes meteor-style async.

Also fixes a mongo-livedata test to pass the required 2 arguments to update.
Merge branch 'wrap-async' into linker