use 'chrono' instead of 'time' crate

[astraw]

Implements conversion back and forth between
chrono::DateTime and NtpTimestamp and a couple
tests.

This works around
RUSTSEC-2020-0071 by removing the time 0.1 dependency and using
chrono instead.

[scottlamb]

I appreciate the thorough tests!

I picked time because I wanted a lightweight library for formatting the time in debug output, and in the particular application I use retina in I don't care about that security advisory (I know the environment never changes in non-test code), but you're right: this should be a general-purpose library so it shouldn't have security footguns like that. So I'm sold on switching away from time.

Two things though:

• I'm less sold on having the time library we use be part of the interface (the new conversions to/fro chrono::DateTime<TZ>. I'm not in love with chrono's API and suspect it will change a bit in the future. I don't particularly want to bump retina's major version every time. Could we maybe compromise on something like UNIX_EPOCH.wrapping_add_secs_and_nanos(secs: u64, nanos: u32) that does the awkward rounding for you but doesn't expose that dependency? (Roughly inspired by https://doc.rust-lang.org/std/time/struct.Duration.html#method.new)
• Can you commit the Cargo.lock changes? Otherwise CI won't be deterministic, ability to build older versions may bitrot over time, and folks will get a dirty working copy as soon as they run any cargo operations.