Too verbose logging when performing arithmetic in full-eviction mode on non-existent key

If an arithmetic operation is performed on a key which isn't resident, on a full-eviction bucket then the following message is often printed in the logs:

2016-11-24T14:48:00.809430Z WARNING (default) Failed to fetch data from database, vBucket=0 key=Key_19 error=document not found [none]

This is unnecessarily verbose - this is not actually a warning in this case and can quickly fill up the logs.

(Note: the reason this is shown is that prior to full-eviction, all metadata would be resident and hence it would be unexpected to attempt to read from disk a key which doesn't exist). However this is no longer the case).