max_purgeable was being incorrectly calculated because the code
that creates vector of uncompacted sstables was wrong.
This value is used to determine whether or not a tombstone can
be purged.
Operand < is supposed to be used instead in the callback passed
as third parameter to boost::set_difference.
This fix is a step towards closing the issue #676.
Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>
b7d36af26f