Prevent Timestamp comparison mistakes in the shell

It's easy to think that Timestamp objects are comparable in the shell:

> Timestamp(1, 9) > Timestamp(1, 8)

true

But they are not:

> Timestamp(1, 9) > Timestamp(1, 10)

true

I don't know what the rules are for comparing Timestamps, but they're not correct. This leads to bugs like SERVER-41008. Let's either fix comparison, or make comparison operators throw an error so we remember to use timestampCmp from shell/utils.js.