Make LevelDB sample keys fixed-length to ensure correct sort-order.

The default LevelDB comparator sorts not only lexicographically, but also by 
key length (which takes precedence). Thus, all variable-length fields must be 
eliminated from the key to ensure the expected lexicographical key sorting.

Note from @matttproud: This reality was unexpected and was exercised in an
explicit test, but apparently the test was not sufficiently thorough.  It is
likely that the rowkey format will be re-evaluated again down the road due to
*not* wanting to write a custom comparator and expand the requisite amount of C
*and* C++-level bindings.