Expose `step` of `tcbdbdefrag` & rename `version.ml` to `camltc_version.ml`

These commits allow to pass a `step` value to `tcbdbdefrag` in order to allow *online defrag* in Arakoon, and rename the generated `version.ml` file to `camltc_version.ml` so Arakoon versions pre-1.7 can still build against the latest versions of this library.