dir/dircache: log any event that wipes the cache
dir/dircache: log any event that wipes the cache

Change-Id: I58521a8635f48d159b7b2722de61f19e924f6777
Reviewed-on: https://upspin-review.googlesource.com/17620
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

upspin/upspin,upspin/upspin