HBASE-30327 Implement adaptive parallel seek in StoreScanner

[ichsansaid]

Summary

Introduces an adaptive parallel seek strategy in StoreScanner that gracefully handles thread pool saturation by dynamically switching between parallel and sequential execution based on available capacity.

JIRA: https://issues.apache.org/jira/browse/HBASE-30327

Problem

When parallel seek is enabled, all StoreFileScanner seek operations are submitted to the RS_PARALLEL_SEEK thread pool with an unbounded queue. Under high concurrency, this can lead to:

• Queue buildup while scanner threads wait for their tasks
• Increased latency from blocking on full queues
• Thread starvation

Solution

Adaptive parallel seek checks pool capacity before submission using a conservative approach (queue.isEmpty() && activeCount < poolSize). When the pool is saturated:

• Falls back to sequential seek on the calling thread
• Re-checks capacity after each sequential seek
• Opportunistically parallelizes when slots become available

Changes

• Add private getAvailableParallelSeekCapacity() in StoreScanner (keeps change local)
• Add adaptiveParallelSeek() method with proper error handling:
  • Inline seek IOException waits for submitted handlers before propagating
  • Uses direct scanner.seek() for inline path (not ParallelSeekHandler)
  • Capacity counted for StoreFileScanners only
• Add configuration hbase.storescanner.adaptive.parallel.seek.enable (default: false)
• Add adaptiveParallelSeekEnabled field to ScanInfo

Configuration

Property	Default	Description
hbase.storescanner.adaptive.parallel.seek.enable	false	Enable adaptive parallel seek (requires parallel.seek.enable=true)

Testing

• Unit tests for adaptive behavior (sequential fallback, mixed mode, error propagation)
• Existing parallel seek tests unaffected