this commit updates our tower dependency from 0.4 to 0.5.

note that this commit does not affect the `tower-service` and
`tower-layer` crates, reëxported by `tower` itself. the `Service<T>`
trait and the closely related `Layer<S>` trait have not been changed.

the `tower` crate's utilities have changed in various ways, some of
particular note for the linkerd2 proxy. see these items, excerpted from
the tower changelog:

- **retry**: **Breaking Change** `retry::Policy::retry` now accepts `&mut Req` and `&mut Res` instead of the previous mutable versions. This
  increases the flexibility of the retry policy. To update, update your method signature to include `mut` for both parameters. ([#584])
- **retry**: **Breaking Change** Change Policy to accept &mut self ([#681])
- **retry**: Add generic backoff utilities ([#685])
- **retry**: **Breaking Change** `Budget` is now a trait. This allows end-users to implement their own budget and bucket implementations. ([#703])
- **util**: **Breaking Change** `Either::A` and `Either::B` have been renamed `Either::Left` and `Either::Right`, respectively. ([#637])
- **util**: **Breaking Change** `Either` now requires its two services to have the same error type. ([#637])
- **util**: **Breaking Change** `Either` no longer implemenmts `Future`. ([#637])
- **buffer**: **Breaking Change** `Buffer<S, Request>` is now generic over `Buffer<Request, S::Future>.` ([#654])

the `Either` trait bounds are particularly impactful for us.

* <tower-rs/tower@v0.4.x...master>
* <https://github.com/tower-rs/tower/blob/master/tower/CHANGELOG.md>
* <tower-rs/tower#815>
* <tower-rs/tower#817>
* <tower-rs/tower#818>

this work is based upon #3504. for more information, see:

* linkerd/linkerd2#8733
* #3504

Signed-off-by: katelyn martin <[email protected]>

this commit updates our tower dependency from 0.4 to 0.5.

note that this commit does not affect the `tower-service` and
`tower-layer` crates, reëxported by `tower` itself. the `Service<T>`
trait and the closely related `Layer<S>` trait have not been changed.

the `tower` crate's utilities have changed in various ways, some of
particular note for the linkerd2 proxy. see these items, excerpted from
the tower changelog:

- **retry**: **Breaking Change** `retry::Policy::retry` now accepts `&mut Req` and `&mut Res` instead of the previous mutable versions. This
  increases the flexibility of the retry policy. To update, update your method signature to include `mut` for both parameters. ([#584])
- **retry**: **Breaking Change** Change Policy to accept &mut self ([#681])
- **retry**: **Breaking Change** `Budget` is now a trait. This allows end-users to implement their own budget and bucket implementations. ([#703])
- **util**: **Breaking Change** `Either::A` and `Either::B` have been renamed `Either::Left` and `Either::Right`, respectively. ([#637])
- **util**: **Breaking Change** `Either` now requires its two services to have the same error type. ([#637])
- **util**: **Breaking Change** `Either` no longer implemenmts `Future`. ([#637])
- **buffer**: **Breaking Change** `Buffer<S, Request>` is now generic over `Buffer<Request, S::Future>.` ([#654])

see:

* <tower-rs/tower#584>
* <tower-rs/tower#681>
* <tower-rs/tower#703>
* <tower-rs/tower#637>
* <tower-rs/tower#654>

the `Either` trait bounds are particularly impactful for us. because
this runs counter to how we treat errors (skewing towards boxed errors,
in general), we temporarily vendor a version of `Either` from the 0.4
release, whose variants have been renamed to match the 0.5 interface.

updating to box the inner `A` and `B` services' errors, so we satiate
the new `A::Error = B::Error` bounds, can be addressed as a follow-on.
that's intentionally left as a separate change, due to the net size of
our patchset between this branch and #3504.

* <tower-rs/tower@v0.4.x...master>
* <https://github.com/tower-rs/tower/blob/master/tower/CHANGELOG.md>

this work is based upon #3504. for more information, see:

* linkerd/linkerd2#8733
* #3504

Signed-off-by: katelyn martin <[email protected]>
X-Ref: tower-rs/tower#815
X-Ref: tower-rs/tower#817
X-Ref: tower-rs/tower#818
X-Ref: tower-rs/tower#819