Added a unit test that would buy/sell a small amount of RAM and a large
amount of RAM.  This test showed unexpected behavior due to attempting
to sell over 2^32 RAM.  I updated the sellram action to enable selling
more than 2^32 bytes at once.

Test results show that for both large and small quantities, the user
losses a small amount of tokens and does not profit.

(cherry picked from commit 10273fa2d180f81769516b507bfdff68f06b7f87)