Shared RocksDB block cache manager

Manages a shared block cache used by all RocksDB instances. This improves memory efficiency and cache hit rates by sharing a single cache across document stores and attachment stores.

The cache is configured via application environment:

   {barrel_docdb, [
       {block_cache_size, 536870912},  %% 512MB default
       {block_cache_type, clock}       %% clock (default) or lru
   ]}

Clock cache provides better performance under concurrent load.