Ignore eviction of files of zero size on EOS

Introduction

As shown in #ops-864, stagerrm is reporting issues when deleting files of dimension zero (garbage collection). Instead, we could simply ignore these files when evicting, because they occupy no space.

Proposed solution

Do not stagerrm zero-length files as there is no space to be gained from GC'ing old 0 length files: file_size_bytes=0.
If the two locations do not match (as seen in #ops-864) then generate an error message/alarm so that an operator can take the appropriate cleanup action.
Check #ops-864 for the complete discussion.