Change get_or_try_insert_with to return a concrete error type rather than a trait object

[tatsuya6502]

Change get_or_try_insert_with of sync::{Cache, SegmentedCache} and future::Cache to return a concrete error type rather than a trait object.

Before (sync::Cache)

pub fn get_or_try_insert_with<F>(&self, key: K, init: F) ->
    Result<V, Arc<Box<dyn Error + Send + Sync + 'static>>>
where
    F: FnOnce() -> Result<V, Box<dyn Error + Send + Sync + 'static>>

After

pub fn get_or_try_insert_with<F, E>(&self, key: K, init: F) -> Result<V, Arc<E>>
where
    F: FnOnce() -> Result<V, E>,
    E: Error + Send + Sync + 'static,

Behavior Change

There is a subtle change in the behavior. get_or_try_insert_with used to guarantee that the closure F to evaluate once even if multiple threads call it at the same time on the same cache key.

Now this is not always true:

• Only one of the closures must be evaluated if the closures in all concurrent calls on the same cache key return the same concrete error type.
• One closure of each error type should be evaluated if the closures in concurrent calls on the same cache key does not return the same concrete error type.

[tatsuya6502]

Merging.