fileHandles are lazy when they could be eager, and fileID to name maps are not cleared appropriately.

[GoogleCodeExporter]

Summary kind of says it all, but we only purge file handles when we bump up 
against the open file handle limit. That is, we only clear their dirty pages 
when absolutely necessary. In the LSM case this is suboptimal, as there really 
aren't very many dirty pages in the disk buffer cache at any one given time. 

Similarly, the map between fileId and FileHandle is not cleared properly 
either, even if the file is closed. We only get rid of an entry in that map if 
a file is actually deleted. 

Original issue reported on code.google.com by ima...@uci.edu on 31 Jul 2015 at 8:15