Async warp example doesn't run

[allancalix]

Describe the bug
The async warp example server fails to build.

To Reproduce

# Tested revision
git checkout aedb2d1e
cargo clean
cd examples/warp-async
cargo run

Expected behavior
An example graphql server should successfully build and start serving on port 8080.

Additional context

• Build issue

error[E0433]: failed to resolve: could not find `document_join_macro` in `futures_util`
   --> /Users/allancalix/.cargo/registry/src/github.com-1ecc6299db9ec823/futures-0.3.1/src/lib.rs:504:15
    |
504 | futures_util::document_join_macro! {
    |               ^^^^^^^^^^^^^^^^^^^ could not find `document_join_macro` in `futures_util`

• The warp version specified in the example manifest is a different major version from the example itself causing issues.

• Some interface changes

I fixed these issues on my fork:
allancalix#1