feat: Implement allocator trait with example implementations

[pzittlau]

This pull request introduces the ability to select different memory allocation strategies for segments within the database. It adds several allocator implementations and a few benchmarks to compare their performance.

Motivation:

• Performance Optimization: By providing different allocation strategies, users can choose the allocator that best suits their workload, potentially leading to improved performance.
• Flexibility: The pluggable architecture allows for easy addition of new allocation strategies in the future.
• Benchmarking and Analysis: The included benchmark suite provides a way to objectively compare the performance characteristics of different allocators.

Key changes:

• Pluggable Allocators:

  • Implemented a generic Allocator trait to define the interface for segment allocators.
  • Added the following allocator implementations:
    • FirstFit: Allocates the first available block large enough.
    • NextFit: Starts searching for free space from the position of the last allocation.
    • BestFitSimple: Allocates the smallest available block that fits the request.
    • WorstFitSimple: Allocates the largest available block.
    • SegmentAllocator: The original allocator, now available as one of the choices.
  • Introduced an AllocatorType enum to configure which allocator to use.
  • Updated the Handler to store Box<dyn Allocator> instances, allowing for dynamic selection of allocators.
  • Added an allocator field to DatabaseConfiguration to specify the desired allocation strategy.

• Benchmarks:

  • Added a new benchmark suite in benches/allocator.rs to evaluate the performance of different allocators.
  • The benchmark uses both uniform and Zipfian distributions for allocation sizes to simulate different workload patterns.
  • It measures the performance of allocation and deallocation operations.
  • Added the zipf crate as a dependency for generating Zipfian distributions.

• Refactoring:

  • Refactored the original SegmentAllocator implementation into its own file and made it one of the selectable options.
  • Moved all allocator-related code into the src/allocator/ directory.

Usage:

To select a specific allocator, modify the allocator field in your DatabaseConfiguration:

let config = DatabaseConfiguration {
    // ... other configurations
    allocator: AllocatorType::BestFitSimple,
    // ...
};

Proposal

Lastly I propose making the NextFit allocator the default one for now. As per the provided benchmarks it's way faster (~100x on my machine) than the original SegmentAllocator.