RUST-1370 Only check specific fields for hello equality

[abr-egn]

No description provided.

[isabelatkinson]

It looks like the only place we were using our PartialEq implementation for HelloCommandResponse was the deleted code on line 151. I'm not sure the reasoning behind the way it was implemented the first time around, but I would guess it was custom logic for server description equality checks so I think we'd be fine to revamp that impl rather than add a new method for equality checking. We'd just want to add a comment similar to the one you added in this PR. As a side note, something I happened to discover today was this pattern:

#[derive(Derivative)]
#[derivative(PartialEq)]
struct A {
    pub x: i32,
    pub y: String,
    #[derivative(PartialEq = "ignore")]
    pub z: Option<f64>,
}

which might be a little bit more maintainable than a manual implementation of the trait if we're not skipping too many fields.

[abr-egn]

I think the reason it's not using Derivative is because server_type() has non-trivial logic so it's not just a field-by-field compare. I'd like to keep the logic for server description comparison specific to that rather than making it the general PartialEq impl since that seems easy to accidentally use elsewhere.

Good point that the impl was only used in the removed code, deleted it :)