Improve performance of MHR script, addresses BIT-1139.

The MHR script involves a "when" statement which can be expensive due to
the way it clones frames/vals.  In this case, the fa_file record is
expensive to clone, but this change works around that by unrolling only
the necessary fields from it that are needed to populate a Notice::Info
record.  A drawback to this is that the full fa_file or connection
records aren't available in the Notice::Info record when evaluating
Notice::policy hooks for MHR hit notices (though they can possibly be
recovered by using e.g. the lookup_connection() builtin_function).
Merge remote-tracking branch 'origin/topic/jsiwek/faster-mhr'

* origin/topic/jsiwek/faster-mhr:
  Improve performance of MHR script, addresses BIT-1139.