Add statistics

Linking to the original idea from @nickva in apache/couchdb#3427:

> 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: davisp/couchdb-erlfdb@187b585 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.