Add a crate feature sync for enabling and disabling sync caches

[tatsuya6502]

Summary

This PR adds a crate feature sync for enabling and disabling sync caches (sync::Cache and sync::SegmentedCache). When enabled, sync caches will be available for use. This feature is enabled by default.

The sync feature will be especially useful when you use experimental dash::Cache because now you can opt out some dependencies such as crossbeam-epoch and uuid by disabling both sync and future features.

Usage

# This is your Cargo.toml

[dependencies]

# Use dash cache, but no other (sync/future) caches.
# Note: "sync" is enabled by default, so you need to disable the default features.
moka = { version = "x.y.z", default-features = false, features = ["dash, atomic64", "quanta"] }

Changes

New crate feature sync:

• Add the sync feature and make it enabled by default.
• Breaking change: Disabling the default features will disable sync caches.
• Update the CI configurations for the sync feature.

future::Cache changes:

• Breaking change: The iterator type for future::Cache has been changed from sync::Iter to future::Iter.
• Breaking change: The return type of future::Cache::invalidate_entries_if method has been changed from Result<sync::PredicateId, _> to Result<future::PredicateId, _>.

Note that this PR will be merged into the next branch for Moka v0.9.0 release. (Currently, the master branch is for Moka v0.8.x releases)

---

Fixes #140

[tatsuya6502]

Merging into the next branch.