Include Tower feature 'load-shed'

## Feature Request
Please add the tower feature 'load-shed' to cargo.toml

### Motivation
Under heavy load I'm seeing thread counts and memory explode (for some reason when the clients begin timing out).

This isn't prohibited by tower::limit::ConcurrencyLimitLayer as it backs-up the requests behind a semaphore.

I'm also not able to try the rate-limit layer because I'm using serve_with_shutdown() which requires layers implement Clone.

I would like to try the Tower feature tower::load_shed::LoadShed to see if this can alleviate my problem.

### Proposal
Amend the dependency in Cargo.toml for tower to include "load-shed" in list of features.