[dm-cache] switch to using an atomic_t for era policy.  Era's now start at 0.
[dm-cache] Change era policy to handle all map results from the child policy.

We have to do this to avoid issues when recovering from a crash.
[dm-cache] rename some variables in the era policy
[dm-cache] tidy era_load_mapping
[dm-cache] era fluff
[dm-cache] forgot to unlock mutex on error path of era_map
[dm-cache] rename increment_current_era message, to increment_era