Retain assignment should be used when re-enabling table(s)

Region servers are picked in round robin fashion when a table is re-enabled.

This results in poor data locality.

We should use retain assignment when re-enabling table(s).

For initial discussion, see HBASE-6143
Since patches for 0.94 and trunk may turn out to be very different, this JIRA is for fixing the issue in 0.94 branch.

Thanks Lars, Elliot, Vladmir and Stack for providing opinions on this subject.