Part#2: cleanup:

In the part 1 of the fix, DS-MRR implementation would peek into
the JOIN_TAB to get the rowid filter from

  table->reginfo.join_tab->rowid_filter

This doesn't look good from code isolation standpoint (why should a
storage engine assume it is used through a JOIN_TAB?).

Fixed this by storing the 'un-pushed' rowid_filter in the DsMrr_impl
structure. The filter survives across multi_range_read_init() calls.

It is discarded when somebody calls index_end() or rnd_end() and cleans
up the DsMrr_impl.