[optimization] avoid writing caches multiple times on file delete events

[martinlippert]

The current cache implementation causes the cache files to be written way too often, especially on file delete events.

On file delete events, the SpringSymbolIndex calls every indexer (java, factories, xml) to react to those file delete events. Every indexer then runs a loop through all deleted files and removes that deletes file from the cache individually, sometimes the cache remove file method is called multiple times for different types (like symbols, bean index, etc.). On every call, the cache writes the updated cache to disc.

We should improve this implementation:

• the cache should write the cache to disc only if something has changed (the removed file was actually part of the cache store)
• the indexer should call one method for all files (a bulk delete method) on the cache implementation to remove all the files from the cache store and then store the cache only once. This has to happen for every type (beans, symbols, etc.), but that should be fine, since there are separate cache files for those different types that all need to be updated.