Update the access time of a cached entry immediately after the entry is read

[tatsuya6502]

Fixes #359.

(We will merge this into the main branch after #348 is merged)

Problem

When a cached entry is read (by e.g. the get method) or write (by e.g. the insert method), the last_accessed time of the entry is updated. When time_to_idle of the Cache is set, the last_accessed time is used to determine whether the entry is expired or not.

Up to Moka v0.12.1, last_accessed time was eventual consistent for read operations. So it will not be updated immediately after read, but will be updated sometime later when pending tasks are processed. (Note: It will be updated to the time when read was performed, not to the time when pending task is processed). This was designed so to achieve better throughput as updating last_accessed has a small performance overhead from a synchronous primitive such as AtomicU64 when the default features are enabled, and Mutex when the atomic64 or quanta feature is disabled.

Note that last_accessed time is strong consistent for write operations, and it is updated during the write.

Before v0.12.0, pending tasks were processed periodically (every ~0.3 secs) by a background thread, so the delay after read was not obvious. However we removed the background thread completely from v0.12.0, and the delay becomes obvious.

Fix

Like the write operations, make last_accessed strong consistent for read operations.

[tatsuya6502]

Merging.