Speed up file deletes when max dir files changes.
Speed up file deletes when max dir files changes.

This change massively speeds up the process with which stenographer cleans up
multiple files.  This is especially important on startup, when MaxDirectoryFiles
has changed.  Before, each delete took >100ms.  Now, they happen very quickly.

We've lost some error tracking here, but before we were just printing out errors
anyway... we're still doing that, so we should be good.

google/stenographer,dcode/stenographer,nbareil/stenographer,dcode/stenographer,gconnell/stenographer,gconnell/stenographer,nbareil/stenographer,dcode/stenographer,nbareil/stenographer,google/stenographer,gconnell/stenographer,google/stenographer