Add function to calculate average TTL

[sggeorgiev]

Overview

Adds efficient TTL tracking and average TTL computation to the estore system using a constant-time algorithm. This enhancement allows for quick insights into expiration characteristics without needing to scan all keys.

Changes

• estoreGetAvgTTL(): Returns the average TTL across all items in an estore (sum_ttl / count)
• estoreReduceTTLSum(): Efficiently reduces the total TTL sum based on elapsed time and number of keys, simulating TTL decay over time

Key Features

• O(1) Performance: Both TTL reading and TTL sum adjustment are performed in constant time, regardless of the number of keys
• Accurate Decay Modeling: TTL sum is reduced as time progresses to maintain realistic tracking, without tracking each key individually

Test Coverage

✅ estoreGetAvgTTL() correctly computes average TTL with multiple entries
✅ TTL averages are correctly updated after removing expirable items
✅ estoreReduceTTLSum() reduces TTL sum proportionally based on elapsed time and key count
✅ Ensures TTL sum never becomes negative
✅ Handles edge cases like empty estore or zero elapsed time gracefully