fix(vlog): upgrade discardStats to RWMutex and secure Iterate read path (#19431)

[HarshalPatel1972]

Description

Fixes #19431

This PR resolves a high-frequency data race and associated memory log truncation leak within the Value Log garbage collection subsystem (discardStats).

Technical Root Cause

The discardStats struct embeds a standard sync.Mutex guarding internal state tracking arrays and maps. While execution boundaries like Update and MaxDiscard correctly acquire exclusive ownership via lf.Lock(), the exported Iterate method completely lacked synchronization.

Under intense concurrent write workloads combined with background telemetry, metrics collection, or CLI debugging cycles, external calls to Iterate would traverse lf.nextEmptySlot and slice elements concurrently while a garbage collection thread or compaction task was executing an un-synchronized Update (which triggers slice mutation and sorting operations via sort.Sort). This led to runtime data races, stale memory tracking offsets, and occasionally missing log truncation windows, leaving old value log (vlog) files stranded on disk.

Resolution Strategy

• Upgraded Locking Posture: Swapped the embedded sync.Mutex inside discardStats out for a high-performance sync.RWMutex.
• Read-Gate Optimization: Wrapped the full range-loop traversal inside func (lf *discardStats) Iterate(...) with shared read-locks (lf.RLock() / defer lf.RUnlock()). This allows infinite concurrent telemetry readers to safely scan statistics without blocking each other or starving the system.
• Exclusive Write Boundary: Maintained exclusive lf.Lock() / defer lf.Unlock() boundaries inside Update to cleanly isolate mutations and sorting operations.

Testing & Regression

• Added TestDiscardStats_ConcurrentRace within discard_test.go.
• The test mounts parallel high-frequency loops: 20 worker goroutines continuously firing randomized statistical Update frames, alongside 20 worker goroutines executing overlapping Iterate read scans.
• Validated using the Go standard race detector (go test -race -v ./...), returning 100% clean thread safety boundaries.

Checklist

• Code compiles correctly and linting passes locally
• Tests added for new functionality, or regression tests for bug fixes added as applicable

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