#71 Table redistribution: limit materialization usage

[Michael Paquier]

Now in table redistribution the data fetched from Datanodes to be redistributed is always materialized.
This materialization is only necessary when new distribution is hash or modulo-based. We should disable it for other types of tables as in this case only raw data is sufficient.

As far as the tests have been done, this is not really expected to improve that much the IO of redistribution, but could really make a good deal on CPU.