make handler reference mutable

[Mdrbhatti]

Issue #, if available:
N/A

Description of changes:
this change will allow calling a handler that implements tower/hyper Service trait.

https://docs.rs/hyper/0.14.14/hyper/service/trait.Service.html
https://docs.rs/tower-service/0.3.1/tower_service/trait.Service.html

By submitting this pull request

• I confirm that my contribution is made under the terms of the Apache 2.0 license.
• I confirm that I've made a best effort attempt to update all relevant documentation.

[rimutaka]

@Mdrbhatti This looks like a breaking change. Is there an issue with a prior discussion?

[Mdrbhatti]

@rimutaka there's no related issue. I was previously using this lambda runtime, which also uses the mutable handler. Using that I was able to create a custom tower Service implementation, and call that via the handler. Unfortunately, there are some bugs in that crate, which prevent me from using it.

Nevertheless, I was able to get the same Service implementation working for this crate, but only after making the changes in the current PR. I tested all of the examples here, and didn't have to change anything to get them to work.

[Mdrbhatti]

I did some digging, and found that mut was removed from the handler in this PR. However, I am still not sure why.

[davidbarsky]

I did some digging, and found that mut was removed from the handler in this PR. However, I am still not sure why.

I'm pretty sure that I removed it on a branch, but I don't fully recall why. I think it was that Tower services weren't fully pulling their weight internally, but if I were still involved in this project, I'd probably switch to something that works more gracefully with Tower.

Nevertheless, I was able to get the same Service implementation working for this crate, but only after making the changes in the current PR. I tested all of the examples here, and didn't have to change anything to get them to work.

This is still technically a breaking change, albeit one that I think is appropriate, as the signature of a trait has changed.

[coltonweaver]

I agree with David in this, as it seems to make sense to try and make this a bit more ergonomic.

@rimutaka What are your thoughts on the matter?

I think I'm in favor of merging and including with 0.5

[coltonweaver]

Approving. We can revisit if this turns out to be a contentious decision

[coltonweaver]

Approved. Can revisit if this is a contentious decision.