Add metadata caching interface

[pjbull]

We need a unified interface for caching metadata across cloud providers.

It should enable end user configuration of:

• Storage mechanism (memory, disk, key-value store)
• TTL
• Cache size
• Eviction strategy
• Cache validation strategy

Related issues include:

• Redundant metadata API calls in Azure Blob Storage operations #533
• Feature request: override local cache logic #422
• Caching of the etag property #319
• S3: Relying on load to check file existance if is file/dir causes high error rate on S3 #181
• Speed up is_dir on entries returned by iterdir and glob #176
• Caching file/dir metadata for GSClient #275