[squash] Count mapped docs one by one

Assuming that couch_stats:increment_counter call is cheap (due to ETS
power on backend) this let us to provide more fine grained values for
mapped docs. Doing this in bulk, as it was, leads us to the situation
when counters are telling that some batch of docs get mapped while it's
what is actually happening right now or even mapping failed on the half
of the way by some reason - this is the case when stats will lie us.