Prevent assigning capital gains splits to lots.

They're generated by lots, and when xaccSplitComputeCapGains sees one it grabs the originating lot which naturally doesn't balance.
Divide the value by the rate to get the amount.

Avoids overflowing the amount and is anyway much more realistic:
Value = Amount * price, so a large "rate" means a small price. Since
get_random_rate() is biased to producing large rates (denom is fixed
at 100), it was effectively testing infinitesimal prices.
Balance ENTER with a LEAVE for all exit points.

For xaccLotFill and xaccLotScrubDoubleBalance, anyway.