Update libp2p and tokio

[ljedrz]

A mostly straightforward update to more recent versions of these dependencies.

Filing it as a draft due to the block_exchange::long_get_block mysteriously failing, at least locally.

[koivunej]

Looked into getting ipfs-http to run with the help of tokio_compat_02 in ljedrz#1. The way I understand the state it'll end up to is:

1. tokio-0.3 running as configured in the http/src/main.rs
2. tokio-0.2 running as single threaded from tokio_compat_02
3. all warp work being executed on tokio-0.2
4. therefore all ipfs work (except for background task, swarm spawns) will happen on single threaded tokio-0.2
5. because single threaded tokio-0.2, some ipfs work will fail, like FsStore using spawn_blocking on tokio-0.3
6. spawn_blocking on tokio-0.3 will fail because of there is no tokio-0.3 runtime available on the single tokio-0.2 thread

(emphasis on spawn_blocking on tokio-0.3 because this threw me off a bit, but makes sense: nowhere in the tokio-0.2 path is the tokio-0.3 reactor made available locally to thread, so it will not be found.)

I think we can land this only when seanmonstar/warp#741 is done along with the bytes 0.6 upgrades happening, which may or may not be linked on the warp PR.

[ljedrz]

Further tokio 0.3 work in warp: seanmonstar/warp#753.

[koivunej]

Status of the upgrade seems that we are still awaiting for the warp PR to go through. At the same time there are at least two major PRs ongoing in rust-libp2p 1907 and the larger 1887. Looks like this will be a big one since we need to adapt to the fixed size multihash as well, unless the two work side by side.

[koivunej]

Rebased, superceded in #446.