Extract Network and Interface from utils

[Hoverbear]

We would like to be able to use this code in benchmarks (#109) and other tests but currently there is no way to use Network or Interface from non-integration test code.

This introduces a harness feature which exposes harness::{Interface, Network}.

There is some rationale to how this was done:

• We cannot have a module in tests/ that is built for benchmarking, so they were moved into a test harness module.
• We cannot use #[cfg(test)] or #[cfg(debug_assertions)] to have this only build in test or development, since bench marking builds in release. So we have to make a feature flag.
• We cannot control which features are used during cargo test or cargo bench by default without altering all defaults (such as what users get when they import Raft). So we must make this a default feature.

Benefits:

• Opens the harness to users in their application code for exploring or testing.
• Opens the harness to benchmarking and unit tests.
• Allows Network and Interface to have their own modules as per Module shuffling and organizing #125 .

Detriments:

• Users may get confused, but the documentation is quite explicit that it is simulator code.
• It will slightly increase non cargo test compile time. The impact should be minimal.
• Users may wish to disable the harness, which would require manual work on their end.

[hicqu]

Make harness as a default feature is not very friendly for user. For example, TiKV needs to disable the feature when declar the raft dependency. So how about make harness as a sub crate?

[Hoverbear]

@hicqu I think "need" is a very strong word, the only reason for TiKV to disable the feature is if we were worried a few milliseconds of compile time.

I think the idea of using a component is very interesting. I will explore this. :)

[Hoverbear]

@hicqu Your idea was very good I think! PTAL again?

[hicqu]

LGTM. Please fix the conflict. 👍

[Hoverbear]

@hicqu Sorry for the delay. :)

[Hoverbear]

@BusyJay PTAL! :) Resolved.

[Hoverbear]

PTAL @hicqu @AndreMouche :)

[AndreMouche]

Rest LGTM

[Hoverbear]

Merging with 2 LGTM from @hicqu and @AndreMouche (they said LGTM but seems it not approve it in the Github UI)