Compute interpolated scores only once.

Before, the smoothing parameters were computed when a linear interpolation
model was constructed, but trigram probabilities were computed and smoothed
on the go (with caching).

We now take a different approach: we compute the smoothed probability over
every known n-gram during construction. After construction, the model then
uses a simple back-off. This gives exactly the same results, but avoids the
need of a hash table that requires locking.