Reduce the number of open file handles used by fisheye

Fisheye currently holds a lot of file handles open, and this seems to have gotten worse in recent releases.

The baseline seems to be around 500 handles for jars, plus 30 per non-passivated repository (up to 100), and then normal transient things consume more on top of this. Lucene indexes are the main issue here, particularly when the merge procedure breaks down and more than 10 files per index are kept. We suspect that because merging indexes can require a lot of file handles once we start running low the indexes have more trouble merging, which accelerates the process of running out of handles.

The workaround for linux system experiencing the "Too Many Open Files" error is described in this knowledge base article.