Gaps handling for global subscriptions

[gschuager]

This is intended to solve #222.

Added gap detection and tombstone creation for PostgreSQL subscriptions:

Gap Detection:

• Monitors sequential global positions during polling
• Detects when next event position > expected (start + 1)
• Configurable timeouts to avoid indefinite blocking

Optional tombstones creation:

• Creates placeholder events at gap positions using try_insert_tombstone SQL function
• Inserts into special __tombstones__ stream with $tombstone message type
• Automatically filtered out from user handlers

Configuration Options:

• GapAgeThresholdMs: Ignore old gaps (default: 1 hour)
• GapSkipTimeoutMs: Max wait time for gaps (default: 5 seconds)
• GapHandlingTimeoutMs: When to create tombstones (default: disabled)

Benefits

• Prevents event skipping in high-concurrency scenarios
• Configurable and backward compatible (opt-in)