Investigate why bucket cache filling up in file mode in an exisiting file is slower

This issue was observed when we recently did some tests with SSD based bucket cache. Similar thing was also reported by @stack and danielpol while doing some of these bucket cache related testing.
When we try to preload a bucket cache (in file mode) with a new file the bucket cache fills up quite faster and there not much 'failedBlockAdditions'. But when the same bucket cache is filled up with a preexisitng file ( that had already some entries filled up) this time it has more 'failedBlockAdditions' and the cache does not fill up faster. Investigate why this happens.