UX Improvements / Optimizations

[gakonst]

• Replace finalizeExits() with finalizeExits(uint64[] slots) which will call finalizeExit on each slot. This allows a user to finalize the exit of multiple of their coins. Allows us to simplify the cleanupExit modifier, and reduce gas costs. Will also omit the getExitIndex function.
• Add timestamp to getExit
• Add cancelExit(uint64 slot) and cancelExits(uint64[] slots) functions. This must check if there is an exit for the specified coin. If yes, it will reset the coin's state to DEPOSITING and free the exitor's bond.
• Add max value to be held by the chain (as a safety measure)
  Notes to self:
• Investigate if it's safe to remove parent coin from challengeBefore.
• Investigate if returning to sequential coin numbering is better