Recheck cutoffScore during phrase_suggest merge.

The goal is to throw out suggestions that only meet the cutoff in some
shards.  This will happen if your input phrase is only contained in a
few shards.  If your shards are unbanced this rechecking can throw out
good suggestions.

Closes #3547.
Force default codec in tests where we rely on the latest lucene defaults
Execute listeners on current thread if threadpool is shutting down