FR: Add valueOf() to Timestamp instances

[dconeybe]

This is a clone of firebase/firebase-js-sdk#2632.

[REQUIRED] Describe the problem

Unable to compare Timestamp instances with native comparators. Javascript calls valueOf() on the object and uses the results of that to do native comparisons.

Steps to reproduce:

Attempt to compare Timestamps directly.

Relevant Code:

firebase = require('firebase-admin')
a = new firebase.firestore.Timestamp(1, 1)
b = new firebase.firestore.Timestamp(2, 2)
console.log(a < b) // false
console.log(b < a) // false

Workaround

Of course you can simply call toMillis() and compare those, but having it auto handled would be better.

[dconeybe]

I've opened this ticket to apply the fix for firebase/firebase-js-sdk#2632 here in this repository also.