Initialize op objects with all properties

[avital]

This way, op objects won't change form later when properties
are assigned. This makes the objects be optimized as structs
in V8.

[coveralls]

Coverage increased (+0.02%) to 96.132% when pulling ff4f80f on avital:make-ops-behave-like-structs into 3d12089 on share:master.

[nateps]

:-)

[nateps]

Actually I just realized the issue with this (and probably why I didn't do it originally) is that this causes Mongo to persist the keys set to null. That could end up taking some more space in Mongo, and we are only adding these properties for pub sub. Instead of this, we should probably copy the appropriate properties from the op and make a new object instead of mutating this object in publish.