Put off 'use' of child classes in Qpsmtpd::DB

Add tests for Qpsmtpd::DB

Remove outdated comment

Qpsmtpd::DB::File::DBM::new()

Qpsmtpd::DB::new() calls Qpsmtpd::DB::File::DBM::new(); if it's absent,
it calls itself recursively instead

Test greylisting against all available stores
Qpsmtpd::DB::Redis simplifications
Tests for Qpsmtpd::DB::Redis
Merge branch 'redis' of github.com:jaredj/qpsmtpd into redis