Refactor configs of read/write cache

[waynexia]

What type of enhancement is this?

Configuration

What does the enhancement do?

Two major file caches in DB are

• read cache, belongs to storage
• write cache, belongs to mito2

And the write cache is marked as "experimental", which should be stable now. This ticket proposes to change all file-related cache configurations to

• storage.cache_path, default to data_home
• storage.cache_capacity, default 10GB. Includes both write cache and read cache.
• (optional) storage.read_cache_capacity which will override storage.cache_capacity
• (optional) storage.write_cache_capacity which will override storage.cache_capacity

Two optional cache capacities are for advanced users who want to fine-tune their cache usage. Capacity set to 0 stands for disabling the cache.

Implementation challenges

This requires a code refactor to extract the write cache from mito engine.