These platforms does not provide `std::sync::atomic::AtomicU64`.

- Add a cargo feature "quanta" and make it a default feature.
- Add a module `common::time` and conditionally choose the source file
  from "time_quanta.rs" or "time_compat.rs".
    - time_quanta.rs file has type aliases to `quanta::{Instant, Clock, Mock}`.
      It also has `AtomicInstant` holding an `AtomicU64`.
    - time_compat.rs file has `Instant` based on `std::time::Instant`
      and quanta-like `Clock` and `Mock`. It also has `AtomicInstant`
      holding a `RwLock<Option<Instant>>`.
- Update Cache, etc. modules to use `common::time` instead of `quanta`.
- Add a step to GitHub Actions to test future feature without quanta/
  `AtomicU64`.

Remove some old codes.

Remove target_arch = "mips" from the cfg_attr for the time module. (It will
not work when quanta feature is still enabled it Cargo.toml)

Add a section to the README to explain that `default-features = false` will
be needed on these platforms.

- Rename the Cargo feature "quanta" to "atomic64".
- Update the change log and readme.
- Update a comment in the yml for GitHub Actions.

Ensure to set num threads to 1 for thread pools when `num_cpus::get()` returns 0.