Calculates the client write buffer size from the number of region

The default size is ~2Mb. There are two issues:
1) It has not changed for a few years, while the memory available has increased
2) It's a fixed size, that does not take into account the number of region.

I think we should have something like 100Kb per region, with a max at 6Mb, all this configurable of course.

I plan to do this after HBASE-6295