Improve Fess Ingest feature implementation

[marevol]

…st coverage

This commit introduces comprehensive improvements to the ingest module:

IngestFactory improvements:

• Replace array-based storage with ArrayList for better performance
• Add caching mechanism with volatile array for efficient concurrent reads
• Implement automatic deduplication (same class replacement)
• Add new methods: remove(), removeByClass(), clear(), size()
• Improve thread-safety with proper synchronization
• Add null validation and defensive copying
• Enhance logging with priority information

Ingester improvements:

• Add priority validation with MIN/MAX/DEFAULT constants (0-999 range)
• Implement equals() and hashCode() based on class type
• Add toString() for better debugging
• Improve Javadoc with detailed parameter descriptions
• Add IllegalArgumentException for invalid priority values

Test coverage improvements:

• IngestFactoryTest: Add 12+ comprehensive test cases covering:
  • Sorting, deduplication, null handling
  • Remove operations, clear, size
  • Defensive copying, concurrent scenarios
• IngesterTest: Replace placeholder tests with 15+ real test cases:
  • Priority validation and edge cases
  • All process() method variants
  • equals/hashCode contracts
  • Custom processing implementations

These improvements enhance code maintainability, reliability, and make the ingest module more robust for production use.