API for compact-in-place operation

It would be convenient to have an explicit API for compacting the database in place, i.e. to the same file. This is what auto-compact does, but if auto-compact isn't enabled, or if the caller wants to run it immediately instead of on a schedule, then the caller has to use fdb_compact, which compacts to a separate file.

I assume the workaround is to compact to a temporary file, then replace the original file with the temporary. But this is several more steps. Since forestdb already contains the logic to compact in place, it'd be convenient if calling fdb_compact(handle, NULL) would do that.