Add stats to erlfdb

It would be nice have erlfdb emit extra stats such as counts for each types of error codes, how many times transactions retry, number of started transactions and some others.

@davisp already has a handy-dandy branch with some stats kept in C++ using some of the newer atomic features: https://github.com/davisp/couchdb-erlfdb/commit/187b585b0bc7c2bcc27fd875f3d59e54a9c028ac so one idea is to expand it.

The other idea is to depend on `couch_stats` and add some of the couch_stats metrics in erlang. That might be easier but the dependency on `couch_stats` might be a bit gross.

Start with the linked couchdb-erlfdb commit and compare its C++ statistics approach with the proposed couch_stats dependency. Done means agreeing on an approach and adding metrics for error-code counts, transaction retries, started transactions, and other selected erlfdb activity.

Written by the indexing model from the issue text.