vinyl: store range lower and upper bounds on disk

Rename range->min_key to range->begin, as it actually denotes not the
minimal key across all entries in the range, but the lower bound of the
range, and introduce range->end for the upper bound of the range.
For adjacent ranges left->end == right->begin. If a range is leftmost,
then range->begin == NULL. If a range is rightmost, then range->end ==
NULL. Store range->{begin,end} in range file on checkpoint and load them
on recovery.

This is required by the following patch to check that ranges do not
intersect.