Speed up phrase suggestion scoring

Two changes:
1.  In the StupidBackoffScorer only look for the trigram if there is a bigram.
2.  Cache the frequencies in WordScorer so we don't look them up again and
again and again.  This is implemented by wrapping the TermsEnum in a special
purpose wrapper that really only works in context of the WordScorer.

This provides a pretty substantial speedup when there are many candidates.

Closes #5395