Store metadata in alternative structure to increase initialization performance.

[Quanhua-Guan]

When more and more data cache in DiskCache, the startup can cost a lot of time, and before that we can not get data from disk cache.

[Quanhua-Guan]

Test on iPhone 8 Plus

Item count Cached | time used to init PINDiskCache

• 228 | 24~27ms
• 1000 | 30~32ms
• 10000 | 123~125ms
• 100000 | 3651ms = 3.651s

[garrettmoon]

@Quanhua-Guan the disk cache initializes asynchronously in part to address these issues, but you're right, there may be a way to increase startup performance by storing metadata in a different place – sadly this would require a major rewrite of the disk caching layer.