remember range endpoints

The Loose Index Scan optimization keeps track of a sequence
of intervals. For the current interval it maintains the
current interval's endpoints. But the maximum endpoint was
not stored in the SQL layer; rather, it relied on the
storage engine to retain this value in-between reads. By
coincidence this holds for MyISAM and InnoDB. Not for the
partitioning engine, however.

Fixed by making the key values iterator 
(QUICK_RANGE_SELECT) keep track of the current maximum endpoint.
This is also more efficient as we save a call through the
handler API in case of open-ended intervals.

The code to calculate endpoints was extracted into 
separate methods in QUICK_RANGE_SELECT, and it was possible to
get rid of some code duplication as part of fix.