TieredMergePolicy needs to take into account the maximum segment size when computing the allowed number of segments

This is a pre-existing issue that was made more likely by LUCENE-7976, given that segments that are larger than the max segment size divided by 2 now are potential candidates for merging: when computing the allowed number of segments, TieredMergePolicy multiplies the level size by maxMergeAtOnce until it finds a level that isn't full. It currently assumes that the level size is always less than the maximum segment size, which might not always be true. This might lead to underestimating the allowed number of segments and in-turn causing excessive merging.