ConnectionManager handling

[carl-berg]

As an attempt to work around #50 i attempted to use the #51 solution in my own custom field -implementation but it is quite hard to override some of GWM's behaviors (interal/private stuff) that it eventually proved unfeasible to do (without copying alot of GWM's code). Some pain points:

• TransactionManager (in a file named ConnectionManager 😄 ) is internal can therefore cannot be replaced
• MigrationBase._connectionManager is private and and gets set like this

        public async Task ManageGalacticWaste()
        {
            using (var connectionManager = ManageConnection())
            {
                _connectionManager = connectionManager;
                await ManageWaste();
            }
        }

...which makes it quite hard to override

I think if maybe a TransactionManager was passed into ManageGalacticWaste (and maybe ManageWaste method aswell) it would simplify overriding stuff. In a longer term maybe MigrationBase could be replaced with a service or something instead of a somewhat clunky inheritance