Replace lazy_static with once_cell

[Razican]

Motivation

once_cell is a macro-free alternative to lazy_static. Its Lazy type has the same API as lazy_static, but it will not use any "macro magic" and it might be more clear to read by non-experienced Rust developers.

This solution is being proposed to be implemented at the Rust standard library level with an RFC. This could be an opportunity to migrate to the new solution and to check that everything is working correctly in Tokio.

Furthermore, as per this comment, it seems that due to a possible compiler bug, using once_cell is noticeably faster than lazy_static in certain hot get operations.

Solution

This PR replaces all mentions to lazy_static by once_cell using the built-in Lazy type.

[taiki-e]

I'm in favor of this. lazy_static has some other (potential) issues (rust-lang-nursery/lazy-static.rs#150, rust-lang/futures-rs#1485 (comment)).

[carllerche]

This PR is fine as a first step.

However, I am noticing that once_cell has support for parking_lot as an optional feature. Ideally, we could enable that feature when we enable that feature, but cargo does not support this.

Instead, we may consider vendoring once_cell to work around this.