dataMgr:data: rename lowestByDate() and highestByDate().

1) Rename lowestByDate() to lowestBeforeDate().
2) Rename highestByDate() to highestBeforeDate().
3) Modify all references to lowestByDate and highestByDate() because
   of 1) and 2).
4) Add test to Turtle module.

Signed-off-by: Zhengwang Ruan <ruan.zhengwang@gmail.com>