[CBM] Investigate how much extra space temporary index files take up

What is the problem?

In Trinity we have changed SQLite's temporary directory to be inside the archive to avoid cases where customers have no room in /tmp. Ideally we should give customers an estimate of how much space is needed.

What is the solution?

We should investigate how big these files are by referring to the SQLite docs and also experimentally. I suggest we write a script that uses inotify or similar to print out file sizes in the tmp directory and then do a backup of a cluster with a single, quite full vBucket with a static key size.