zdbd: implement index rebuild as internal command

[maxux]

There is a staging code which allows to rebuild index based on data files, but this were never finished.

Related to #159, it will be interesting to support index rebuild directly on zdb runtime, so like that, we can copy a full namespace from one database to another (only by copying data files), then rebuild index and get namespace up and running without anything else to do and without the need to lock master namespace to avoid index changes.