Add BitFaster.Caching

[bitfaster]

Caching/BitFaster.Caching - https://github.com/bitfaster/BitFaster.Caching

High performance, thread-safe in-memory caching primitives for .NET.

Features:

• ConcurrentLru, a lightweight pseudo LRU based on the 2Q eviction policy. Also with time based eviction.
• ConcurrentLfu, an approximate LFU based on the W-TinyLFU admission policy. Also with time based eviction.
• Configurable atomic valueFactory to mitigate cache stampede.
• Configurable thread-safe wrappers for IDisposable cache values.
• A builder API to easily configure cache features.
• SingletonCache for caching single instance values, such as lock objects.
• High performance concurrent counters.