Add support for eviction listener

[tatsuya6502]

This PR adds a support for the eviction listener to the following caches:

• sync::Cache
• sync::SegmentedCache
• future::Cache

The eviction listener is a closure with the following signature and can be used (for example) to keep other data structures in-sync with the cache.

impl Fn(Arc<K>, V, RemovalCause) + Send + Sync + 'static
where
    V: Clone,

and the enum RemovalCause (moka::notification::RemovalCause) is defined as the followings:

#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub enum RemovalCause {
    /// The entry's expiration timestamp has passed.
    Expired,
    /// The entry was manually removed by the user.
    Explicit,
    /// The entry itself was not actually removed, but its value was replaced by
    /// the user.
    Replaced,
    /// The entry was evicted due to size constraints.
    Size,
}

The enum DeliveryMode specifies how and when an eviction notification should be delivered to an eviction listener.

#[derive(Copy, Clone, Debug, PartialEq, Eq)]
pub enum DeliveryMode {
    /// With this mode, a notification should be delivered to the listener
    /// immediately after an entry was evicted. It also guarantees that eviction
    /// notifications and cache write operations such and `insert`, `get_with` and
    /// `invalidate` for a given cache key are ordered by the time when they
    /// occurred.
    ///
    /// To guarantee the order, cache maintains key-level lock, which will reduce
    /// concurrent write performance.
    ///
    /// Use this mode when the order is more import than the write performance.
    Immediate,
    /// With this mode, a notification will be delivered to the listener some time
    /// after an entry was evicted. Therefore, it does not preserve the order of
    /// eviction notifications and write operations.
    ///
    /// On the other hand, cache does not maintain key-level lock, so there will be
    /// no overhead on write performance.
    ///
    /// Use this mode when write performance is more important than preserving the
    /// order of eviction notifications and write operations.
    Queued,
}

Currently, sync caches support both modes and Immediate mode is the default. But future cache supports only Queued mode.

---

Fixes #128

[tatsuya6502]

Merging.