Chord and Snapshot mappers should drop facts not in dictionary.

Fixes #381.

This change also introduces a set of dropped counters for
visibility into what is being dropped for debugging purposes,
_however_ given that this is a fairly standard situation in
the future we should consider moving it behind a debug flag
or similar to avoid polluting debugging with noise.

Ready for review once tests pass /cc @domdere @nhibberd @charleso