Prevent drift in tick delivery (fix #406)

[drrlvn]

Calculate the next delivery time based on the previous one instead of the time the thread awoke, preventing a small but noticeable drift from accumulating.

[drrlvn]

I fixed the tests so they now pass. I don't know why the CI fails on 1.26/1.28, maybe it's a dependency issue. This is the error:

error[E0432]: unresolved import `self::std::hint`

  --> /home/travis/.cargo/registry/src/github.com-1ecc6299db9ec823/lazy_static-1.4.0/src/inline_lazy.rs:13:16

   |

13 | use self::std::hint::unreachable_unchecked;

   |                ^^^^ Could not find `hint` in `std`

error: aborting due to previous error

[drrlvn]

I don't know why the CI fails on 1.26/1.28, maybe it's a dependency issue.

Happens on master as well, see #412.

[ghost]

Hey, thank you for submitting this PR and apologies for coming to it so late! :( I'm assuming you already forgot about this code so I submitted a new PR: #456

I think try_recv() should not change - the drift there is intended and this is exactly how tickers work in Go, too. However, we do need changes in recv() - instead of sleeping and then attempting to receive a message, we should receive it first and then sleep.

[drrlvn]

@stjepang OK, great, I'll close this PR then. Thanks!