Refactor reactive dict into a serializing and non-serializing version

[stubailo]

• Until now, ReactiveDict always serialized and deserialized everything when getting and setting. This means you can't store any values that are not serializable, for example functions and objects with prototypes
• With this PR, the new default is to not serialize
• If you pass a string to the constructor, ReactiveDict will give you back a SerializingReactiveDict to preserve backwards compatibility with migrating ReactiveDict (like Session)
• You can also call the SerializingReactiveDict constructor directly
• ReactiveDict#equals doesn't work on a non-serializing reactive dict, because it is impossible to implement it efficiently

Breaking changes

• If you construct a ReactiveDict with no argument to the constructor, you will not be able to use equals

Possible solution

• Keep ReactiveDict as it is, and introduce a new class called NonSerializingReactiveDict, but that seems a little silly

[dgreensp]

equals is only specced to work on primitives, so what's the problem?

Incidentally, it's now possible to make SerializingReactiveDict#equals work efficiently on all EJSON types, because EJSON.stringify has a canonical option that it didn't have when this code was first written! (See the comment about how we can only allow primitives because of key order in serialized objects.) We talked about doing that eventually, but one reason to keep equals as is (only working on primitives) would be so that it also works on a non-EJSON ReactiveDict like you are making. It seems like we should do one or the other, though, not have equals work only on primitives and ban it on non-serializable ReactiveDicts.

I think it's confusing for new ReactiveDict to return different classes with different capabilities depending on the number of arguments, unless the string-argument form is deprecated.

How about MigratedReactiveDict instead of SerializingReactiveDict? We could also work the word "session" in there somehow, evoking Session and sessionStorage. ReactiveSession or ReactiveSessionDict. But I think I like MigratedReactiveDict better.

[stubailo]

Going to wait until more product direction on Tracker/reactivity.