Store smtp connection statistics in the database and send authentication attempts info to network intelligence

Ref #532

Please review !767 (merged) first.

Introduces a new database which contains statistics of all incoming smtp connections used by the MUAs that try to authenticate (send smtp AUTH command), either succeeding or not.

Also sends such statistics, alongside with the IP addresses (which might be an issue in terms of privacy!)

This is a big change that also does some refactoring, introducing a new package pkg/dbrunner, which is a Runner that groups database actions in transactions delimited by timeouts.

The code was extracted from deliverydb and is now reused in the new package connectionstats which has a new database with some basic statistics about smtp connections, provided by postfix.