Index management during fetch

[kumekay]

Refactor index management to use fetch-based counters instead of time-based intervals

Implementation details:

• Replace time-based BitmapInterval with counter-based MultiPackIndexInterval and BitmapIndexInterval
• Multi-pack index now refreshes every 10 fetches (default, configurable per repo)
• Bitmap index now rebuilds every 50 fetches (default, configurable per repo)
• Both intervals can be disabled by setting to 0
• Add per-repository override support for both index intervals
• Remove separate bitmap refresh goroutine, integrate all operations into main mirror loop
• Add fetch counter tracking with mutex synchronization per repository

[kumekay]

@vsaltxx PTAL

[kumekay]

@vsaltxx Thank you so much for detailed and valuable review!

[vsaltxx]

@kumekay Thank you for implementing the requesting changes! I left one comment about updating the documentation, it is on you weather to implement it or not :)