Minor updates

[asad-awadia]

This PR introduces changes to CloverDB, focusing on compound indexing, advanced query planning, and a more expressive update API.

Key Changes

1. Compound Indexing Support

• Multi-field Indexes: The indexing system now supports creating, checking, and dropping indexes on multiple fields.
• API Updates: CreateIndex, HasIndex, and DropIndex now accept variadic field arguments.
• Planning: The query planner can now leverage compound indexes for range queries and sorting.

2. Advanced Query Planning & Execution

• Logical OR Support: Implemented unionNode to handle queries with OR criteria efficiently using multiple index scans.
• Index Intersection: Implemented intersectionNode to combine results from multiple independent indexes, improving query performance for complex criteria.
• Sorting Optimizations: Improved index selection for sorted queries.
• On-Disk Sort: Introduced onDiskSortNode for handling large result sets that exceed memory limits during sorting.

3. Rich Update API (Update Operators)

• MongoDB-like Operators: Added support for update operators:
  • Set(field, value): Sets a field to a specific value.
  • Unset(field): Removes a field from the document.
  • Inc(field, value): Increments a numeric field.
  • Push(field, value): Appends a value to an array field.
• Fluent Update API: The Update method now accepts these operators, allowing for atomic-like updates on documents.

4. API Enhancements & Refactoring

• Update Return Values: Update and UpdateFunc now return the list of affected document IDs and the total count, providing better feedback for write operations.
• Document Logic: Added helper methods in document/document.go and util/ for better document manipulation.