Ease mastering requirement to not require consecutive success

Formerly, a session would only exit after consecutively demonstrating
mastery of some number of items. This had the affect of skewing the
set positive, and potentially doing a large amount of review without
introducing any new material.

The original intent of the mastery requirement was simply to provide
some review of previously learned material after introducing new
material. So we now simply require some number of items to be mastered,
but we don't require them to be achieved consecutively, (that is,
we don't reset the count at any miss).