Tundra: only throw errors when ensuring evictable keys

This involves a tradeoff: we need to store the names of every key replicated
to a Tundra datastore.

The advantage is that this allows a more natural API pattern: we can ensure
keys even when we're not sure if they _should_ exist or not.
Tundra: allow configurable message queue name
Tundra: update tests
Tundra unit tests: use wcar* macro
v2.4.0-beta3