refactor: Calculate bucket stats (file_count, size) using files associated to bucket

[HermanObst]

Problem: we are currently calculating Buckets file_count and total_size by increasing/decreasing on file deletion and creation. This method is error prone as it can easily differ to what we actually have in db.
Even more, we were seeing a file_count = U64::MAX in testnet, probably because file_count incorrectly with value = 0 and after a deletion file_count = 0 -1 = U64::MAX

Solution: Calculate file_count and total_size based on the files associated to the bucket