feat: add leaseCache to change gc policy

[Desiki-high]

Change GC policy

This patch changes GC policy from LRU to LFU/LRU: cleans the blob by LFU first then LRU.

Change:

• add singleflight for gc.
• add leaseCache: the cache of lease, help record the used count and time.
• implement the new policy of GC by leaseCache.
• update lease in commit. To make leaseCache is synchronized with local storage before read each blob.

⚠️Notice:
The leaseCache is not thread-safe, we make sure the safe by the gcSingleflight and the thread-safe of content.db.Update.

Ref: #141

Test result of leaseCache:

• TestLeaseCache 0.007s Add(5) Remove(3)
• TestLeaseCacheInit 0.016s Add(6) Remove(1) Init(1) Add (5)