Remove the time dependency where possible

[tomaka]

cc #7915

The time crate is no longer maintained. This PR replaces it with std::time where possible. Wherever strftime is used, the time dependency has been kept because there is no straight-forward replacement yet.

In addition to being IMO a positive change, this should also help compile for wasm.

The logic of the code has been preserved, except in a few places where initializing a u64 to 0 has been replaced with initializing an Instant to Instant::now(). Since the 0 was mostly a place-holder (similar to None), the behaviour should be exactly the same in the end.

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[tomaka]

Note that in the future when const fns are stable, we should replace the various u64 constants with Duration constants.

[tomaka]

After this PR, time is used in the following places:

• In hyper and related crates ; hyper will probably be removed from the client light, because we don't need anything HTTP-related.
• ethstore because it requires strftime to generate file names. It should be possible to tweak something for wasm here.
• zip (and msdos_time) ; used only by the dapps crate which we don't need for the light-client-in-the-browser objective.
• node_health (and its dependency ntp) and logger because they require strftime. This is probably the hardest one to fix.
• rust-crypto ; I should be able to submit a PR there, but the crate is unmaintained so it's probably not worth it
• transient-hashmap ; opened a PR there: Remove the dependency on the time crate debris/transient-hashmap#10
• vergen ; but it's only used as a build-dependency so it's not a problem.

[tomaka]

I just realized that std::time::Instant is not steady, so replacing SteadyTime with Instant could be wrong. However considering that it's only about caching and network timeouts, I don't think that we actually need steady clocks.

[niklasad1]

@tomaka
What about std::time::SystemTime it is not monotonically increasing but steady I think.