Multiple challenges and bonds

[gakonst]

Now that the contracts support multiple challenges, if an exit has N challenges, and X bond, the first challenge will get the whole bond, and potentially the slashBond function in line 390 during the finalization of an exit will revert.

1 possible countermeasure is to just split the slash in N equal parts and award the challengers.
Other approach is to just reward the first challenger

(need to write a test that illustrates this potential exploit to verify its existence, and then a countermeasure).