- Create invalidator module.
- Add thiserror crate to the dependencies.

- Implement iterator for deque.

Add invalidator thread pool

Implementing background invalidation task.

- Implementing background invalidation task.
- Add the following trait bounds to the generic parameters of sync and
  future caches:
    - K: Send + Sync + 'static
    - V: 'static
    - S: Send + 'static

- Implementing background invalidation task.
- Add Send + Sync bounds to a generic parameter V of sync and future
  caches, so K, V and S have the following bounds:
    - K: Send + Sync + 'static
    - V: Send + Sync + 'static
    - S: Send + 'static

- Remove finished predicates after a invalidation scan.

- Simplify the implementation of Predicate

- Tweak a unit test.

- Refactoring on the background invalidation task.
- Add Sync bounds to a generic parameter S of sync and future caches,
    so now all K, V and S have the following bounds:
    - Send + Sync + 'static

- Add the invalidate_entries_if method to the future cache.
- Add the support_invalidation_closures method to the builders in sync and
  future modules.

- Update the get methods in sync and future caches to check if the entry
  has been invalidated.

- Add invalidate_entries_if to the unsync cache.

- Simplify the implementation of invalidate_entries_if in the
  unsync cache.

Conflicts:
    CHANGELOG.md
    Cargo.toml

Write API doc.

- Add dependency to uuid crate and change the predicate ID type from u64 to
  PredicateId, which is String of UUID version 4.
- Remove NoSpaceLeft variant from PredicateError as UUID collision will
  extreamly unlikely happens.
- Rename PredicateRegistrationError to PredicateError.
- Upgrade Quanta crate version from v0.7.1 to v0.8.0.
- Write docs for PredicateId and PredicateError.

- Brush up the docs for unsync::Cache::invalidate_entries_if.

Conflicts:
    CHANGELOG.md
    src/sync/base_cache.rs
    src/sync/cache.rs

- Rename CacheBuilder::enable_invalidation_with_closures to
  support_invalidation_closures.
- Remove an unused field from Invalidator.

- Implement `invalidate_entries_if` on `sync::SegmentedCache`.
- Optimize `base_cache::Inner::submit_invalidation_task` method for
  a case when the write order queue is empty.
- Refactor some cache methods for refactoring.

Fix a deadlock issue for dropping sync/future cache while running background
invalidation task.