Start using constructor injection

[judgej]

Most of the methods supported by the Server (or Direct or Shared) require access to a Transaction model with its relevant storage. That should be moved to the constructor so that we can guarantee that it is available, instead of keep checking whether it has been set whenever we need it.

Any apps that make use of the setter would need modifying, so this is a warning :-)

Similarly, TransactionPdo should also take a database object in its constructor, since it will not work without one.