Add OCR_AGENT_CACHE_SIZE environment variable (#4066)
## Problem
OCR agents used unlimited caching, causing excessive memory usage. Each
cached OCR agent consumes different amounts of memory, but can easily
consume ~800MB.
## Solution
Add `OCR_AGENT_CACHE_SIZE` environment variable to limit cached OCR
agents per process.
- **Default**: 1 cached agent
- **Configurable**: Set to 0 to disable caching, or higher for more
languages
