CDC: Negative fragmentation ratio reported

Peter Searby noticed in his testing that fragmentation ratio is being reported as negative. Also HistoryDiskUsage is going above the ActiveDiskUsage (i.e. db_file_size < history_file_size).

1. Start 1 node cluster with 1024MB quota
2. Add a magma bucket “default”
3. Enabled history with:

curl -u Administrator:asdasd localhost:9000/pools/default/buckets/default -d historyRetentionBytes=5000000 -d historyRetentionSeconds=10000
4.  Start pillowfight with:
./cbc-pillowfight -U couchbase://127.0.0.1:12000/default -u Administrator -P asdasd -v --num-items 10000 -m 5 -M 200