util: bump tokio dependency to 1.6 to satisfy minimal versions

[djc]

This failed in h2 CI:

https://github.com/hyperium/h2/runs/5153518790?check_suite_focus=true

It looks like minimal versions support does not the right thing in the face of path dependencies.

[hawkw]

Looks good to me!

It's really a shame we can't easily test this...

[djc]

Don't we remove the path deps in release prep PRs? Maybe we could somehow make sure it gets checked at that time?

[taiki-e]

Don't we remove the path deps in release prep PRs? Maybe we could somehow make sure it gets checked at that time?

I implemented a flag for this in cargo-minimal-versions (taiki-e/cargo-minimal-versions#4), but other workspace members have higher version requirements, so it seems that implementation of taiki-e/cargo-minimal-versions#1 is also needed to catch problems like the one this PR fixes.

[taiki-e]

I (partially) implemented taiki-e/cargo-minimal-versions#1 in taiki-e/cargo-minimal-versions#4.
And it can catch the problem that this PR fixes, within tokio workspace!

$ cargo minimal-versions check --workspace --ignore-private --detach-path-deps --all-features

...
info: running `cargo check --all-features` on tokio-util (1/1)
   Compiling libc v0.2.86
    Checking cfg-if v0.1.6
   Compiling syn v1.0.0
    Checking log v0.4.8
    Checking mio v0.7.8
    Checking tokio v1.0.1
   Compiling futures-macro v0.3.11
    Checking futures-util v0.3.11
    Checking tokio-util v0.7.0 (/Users/taiki/projects/forks/tokio-rs/tokio/tokio-util)
error[E0432]: unresolved import `tokio::sync::mpsc::OwnedPermit`
 --> src/sync/mpsc.rs:5:5
  |
5 | use tokio::sync::mpsc::OwnedPermit;
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ no `OwnedPermit` in `sync::mpsc`

...

It also found other minimal versions problems.

1. mio requirement in tokio (macos specific):

info: running `cargo check --all-features` on tokio-stream (1/1)
   Compiling libc v0.2.69
   Compiling log v0.4.8
   Compiling autocfg v1.0.0
   Compiling memchr v2.2.0
    Checking cfg-if v0.1.2
    Checking arc-swap v0.4.0
    Checking pin-project-lite v0.2.5
    Checking once_cell v1.5.2
    Checking bytes v1.0.0
    Checking futures-core v0.3.0
    Checking futures-sink v0.3.0
   Compiling tokio v1.8.0
    Checking signal-hook-registry v1.1.1
    Checking mio v0.7.6
error[E0425]: cannot find value `TCP_KEEPINTVL` in crate `libc`
    --> /Users/taiki/.cargo/registry/src/github.com-1ecc6299db9ec823/mio-0.7.6/src/sys/unix/tcp.rs:334:15
     |
334  |         libc::TCP_KEEPINTVL,
     |               ^^^^^^^^^^^^^ help: a constant with a similar name exists: `TCP_KEEPALIVE`
     |
    ::: /Users/taiki/.cargo/registry/src/github.com-1ecc6299db9ec823/libc-0.2.69/src/unix/bsd/apple/mod.rs:2259:1
     |
2259 | pub const TCP_KEEPALIVE: ::c_int = 0x10;
     | ---------------------------------------- similarly named constant `TCP_KEEPALIVE` defined here

UPD: filed #4492 to fix this.

2. proc-macro2 requirement in async-stream-impl:

info: running `cargo check --all-features` on tokio-test (1/1)
   Compiling proc-macro2 v1.0.0
   Compiling unicode-xid v0.2.0
   Compiling autocfg v1.0.0
   Compiling syn v1.0.0
    Checking futures-core v0.3.0
    Checking pin-project-lite v0.2.5
    Checking bytes v1.0.0
   Compiling tokio v1.2.0
   Compiling quote v1.0.0
    Checking tokio-stream v0.1.1
   Compiling async-stream-impl v0.3.0
error[E0277]: the trait bound `TokenStream2: From<proc_macro2::TokenTree>` is not satisfied
  --> /Users/taiki/.cargo/registry/src/github.com-1ecc6299db9ec823/async-stream-impl-0.3.0/src/lib.rs:17:17
   |
17 |     let input = TokenStream2::from(TokenTree::Group(Group::new(Delimiter::Brace, input)));
   |                 ^^^^^^^^^^^^^^^^^^ the trait `From<proc_macro2::TokenTree>` is not implemented for `TokenStream2`
   |
   = help: the following implementations were found:
             <TokenStream2 as From<proc_macro::TokenStream>>

The implementation is still incomplete and there are a few bugs, but (hopefully) I can probably release it by end of this week or next week.