Cargo: optimize feature flags of tokio

[dongsupark]

Disable full feature of tokio, instead enable only necessary features, macros, net, rt-multi-thread.
Doing that, build time drops by ~10%.

See also #18.

[pothos]

Sounds ok but I don't know if, e.g., time should also be there for the schedulers to work correctly?

[wrl]

looks fine to me. do you still want to look into smol to see if it works for our purposes?

[dongsupark]

do you still want to look into smol to see if it works for our purposes?

Yes, I would like to look into smol in the long term.
It seems possible to replace simple code of tokio with smol, at least inside our code base.

The thing is, however, that other libs like reqwest still heavily rely on tokio. The reqwest currently does not support smol. (See seanmonstar/reqwest#1944)
So it is not trivial to get rid of tokio completely.