[BUG] Important bug fix in an optimization.

Reduction(axis!=0) of joins(axis=0) wasn't working on the right axis.

fix gh-2109
Don't generate too much warning (and also fix a failure in travis tests)
remove warning in test.
Add missing RandomStateType.may_share_memory.
Better message.
Merge pull request #2110 from nouiz/bug_reduction_join

[BUG] Important bug fix in an optimization.