Migrate from tokio 0.1 to 0.2

[spl]

At some point, I suppose terminus-store should be migrated from tokio 0.1 to 0.2.

See the following related items:

• https://users.rust-lang.org/t/failed-to-port-mononoke-to-tokio-0-2-experience-report/32478
• https://qiita.com/zenixls2/items/c594c87bf208a7094905
• https://www.ncameron.org/blog/migrating-a-crate-from-futures-0-1-to-0-3/

[matko]

It's definitely on the todo list, but unfortunately not on the short term due to other priorities. Doing the conversion will temporarily block all other development on the library and we simply can't do that right now.

As a rough estimate, I don't expect work on this to be able to commence until march.

[rrooij]

There is by the way a tokio_compat crate which should make the transition to tokio 0.2 easier.

[spl]

Some additional things to keep in mind:

• The bytes interface changed between 0.4 (the dependency of tokio-io 0.1) and 0.5 (the dependency of tokio 0.2). (In Replace AsRef<[u8]> with Bytes, stop using mmap #44, I avoided using Buf and BufMut to keep the usage of bytes simple with the tokio transition in mind.)
• Newer versions of tokio have methods like read_u64 and write_u64 for big-endian numbers (io: read/write big-endian numbers tokio-rs/tokio#1863).

[rrooij]

7c4df74
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