Fix bug, add method Resize

[zhangcunli]

1. add method Resize
   newSize >= oldSize, keep all entry
   newSie < oldSize, discard all entry;

2. bug, case:

package main

import (
	"fmt"
	_ "strconv"
	_ "time"

	"github.com/coocood/freecache"
	//"github.com/zhangcunli/freecache"
)

func main() {
	cachesize := 512 * 1024
	cache := freecache.NewCache(cachesize)

	value1 := "aaa"
	key1 := []byte("key1")
	value := value1
	cache.Set(key1, []byte(value), 0)

	it := cache.NewIterator()
	for {
		entry := it.Next()
		if entry == nil {
			break
		}
		fmt.Printf("-----------> iterator, key:%s, value:%s\n", string(entry.Key), string(entry.Value))
	}

	value = value1 + "XXXXXX"
	cache.Set(key1, []byte(value), 0)

	value = value1 + "XXXXYYYYYYY"
	cache.Set(key1, []byte(value), 0)
	it = cache.NewIterator()
	for {
		entry := it.Next()
		if entry == nil {
			break
		}
		fmt.Printf("-----------> iterator, key:%s, value:%s\n", string(entry.Key), string(entry.Value))
	}
}

[coocood]

*= 2

[coocood]

@zhangcunli
Thank you for your great work.
Would you please add the case to test and make sure all tests are passed?
And we need to test that when size is increased, all old entries are preserved.

[funny-falcon]

Why atomic.AddUint64 ? There is no concurrent access to local variable.

Why this function were changed at all? Can you prove there is a need to change this function?

[coocood]

@zhangcunli any update?

[zhangcunli]

Modify the ringbuffer resize, ringbuffer_test.go:
////////////////////////////////////////////////////////////////////////////////////
ringbuf_test.go:28: [size:32, start:17, end:33, index:0]
ringbuf_test.go:29: ghibbbbccccefghi
size=16 --> size to 64 --> size to 32
Now ringbuffer index should be 16, we should not set the index to 0 and overwirte the ringbuffer;