GCache2: Have a consistent mutex locking policy

[erwanor]

Example:

func (c *ARC) Remove(key interface{}) bool {
	c.mu.Lock()
	defer c.mu.Unlock()

	return c.remove(key)
}

However, for the Get method we do:

func (c *ARC) Get(key interface{}) (interface{}, error) {
	v, err := c.get(key, false)
	if err == KeyNotFoundError {
		return c.getWithLoader(key, true)
	}
	return v, err
}

further down the execution path, we see that the mutex is locked in getValue:

func (c *ARC) getValue(key interface{}, onLoad bool) (interface{}, error) {
	c.mu.Lock()
	defer c.mu.Unlock()

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We should be careful about this. Without a consistent policy about where to lock/unlock a mutex in the execution path it can be easy making a mistake that compromise the concurrency-safe property of the library.