- Make DsMrr_impl::dsmrr_init() handle the case of
1. 1st MRR scan using DS-MRR strategy (i.e. doing key sorting and rowid sorting)
2. 2nd MRR scan getting a buffer that's too small to fit one key element
and one rowid element, and so falling back to default MRR implementation
In this case, dsmrr_init() is invoked with {primary_handler, secondary_handler}
initialized for DS-MRR scan and have to reset them to be initialized for the
default MRR scan.
(attempt 2, with simplified testcase)
