Break interior-node and leaf-node readers apart in loadSegment().

Break interior-node and leaf-node readers apart in loadSegment().
Previously, the code looped until the block was a leaf node as
indicated by a leading NUL.  Now the code loops until it finds a block
in the range of leaf nodes for this segment, then reads it using
LeavesReader.  This will make it easier to traverse a range of leaves
when doing a prefix search. (CVS 3884)

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