feat: implement Timestamp.valueOf() (#944)

[dconeybe]

This enables Timestamp objects to be compared for relative ordering using the <, <=, >, and >= operators.

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Fixes #944

[schmidt-sebastian]

Can you confirm that the backend rejects all older timestamps, as well as any timestamps past the year 10,000?

If that is the case, then we should add validation to the constructor. We need to make sure that valueOf() is never invoked on invalid data. If that is not the case, then we should go back to the drawing board and come up with a strategy.

I am also slightly inclined to ship this as part of the Node 8 deprecation that is around the corner, in which case we can make this a proper breaking change. The PR title would then be "feat!: ...".

[dconeybe]

I tested it out and an exception is thrown when invalid seconds are specified. The message looks like this:

Error: 3 INVALID_ARGUMENT: Timestamp seconds exceeds limit for field.

I wrote a little app to test this out https://github.com/dconeybe/node-timestamp-range-test.

I like the idea of adding the validation to the constructor and shipping this as part of the Node 8 deprecation. I will update this PR with that logic.

We could even go a step further and make timestamp.ts an exact copy of https://github.com/firebase/firebase-js-sdk/blob/master/packages/firestore/src/api/timestamp.ts. There is no reason why these two files (and their accompanying unit tests) should differ in any way. Making them identical would help reduce the maintenance burden. What do you think?