single os_mem_alloc_large() call.  The function mem_alloc() that was
used previously allocates unaligned memory.

row_merge_blocks(): Replace block1, block2, block3 with a single array.
Replace buf1, buf2, buf3 with buf[3].  Replace b1..b3 with b0..b2.
Replace mrec1,2 with mrec0,1.  Replace offsets1,2 with offsets0,1.

row_merge(), row_merge_sort(): Replace block1, block2, block3 with a
single array.