Adding Maintenance Capabilities to PDX

[lkuffo]

New Features

Added a maintenance API for our PDXTree index. The API exposes index.append() and index.delete() methods in both the Flat and 8-bit variants.

• Sustains 40k INSERTS/sec in 1 million scale datasets while maintaining high recall and low search latency thanks to PDX.
• We have implemented an in-memory LIRE protocol (SPFresh) within our Tree index, combining some ideas of Quake. Further boosted by PDX pruning in several instances.

Limitations

• Single threaded writer is assumed
• Fully in-memory implementation