Disable p2p network when delayed_node plugin is enabled

[pmconrad]

Resolves #104

[abitmore]

Better cover network_broadcast_api as well.

[pmconrad]

network_broadcast_api methods already contain null-checks for p2p_node(). Question is if an error reply would be more appropriate than silently dropping the stuff to be broadcasted.
Also, broadcast_transaction_with_callback and broadcast_transaction_synchronous leak memory in this case because the callback is never executed. (The memory still gets free'd when the client closes the connection.)

[abitmore]

Question is if an error reply would be more appropriate than silently dropping the stuff to be broadcasted.

I think raising an error is better.

[pmconrad]

Did some refactoring in database_fixture. This could also be a first step towards #1907 .