ASSERTION: !USES_BLOB_VALUE(TABLE->READ_SET) IN MULTI_RANGE_START_RETRIEVALS

Starting with MySQL-5.6-cluster-7.3.1, (bug 16020945) columns in 'read_set' required due
to implementation specific details of each handler is not any more calculated
(read: 'guessed') by the optimizer.  Instead additional columns may be
added to the read_set by each handler before the table access is
init'ed.

ha_ndbcluster::get_read_set() was introduced for this purpose.

However, the native MRR implementation in Cluster called get_read_set()
too late. Thus we failed to detect Blob colums being part
of the read set, which should have forced is to use use the 
default MRR impl. instead of the native NDB MRR impl..

This caused us to later hit the assert mentioned in the bug synopsis