master: Fix handling metadata.mfs.lock in shadow master

This commit fixes an issue with the shadow master which cause it to mark
a "dirty" working directory as "clear" in the following scenario:

* leave the working directory in "dirty" state,
  e.g. by pkill -9 mfsmaster
* start a shadow master there, but don't let it download any metadata
* stop the shadow master - it would unlock the data dir

The issue is fixed by preventing the shadow master from removing the
metadata.mfs.lock file if it didn't store a valid metadata file.

Change-Id: I2682ca358aa1b884c7dfbcc8a702fa22f53ed5e1