The seeder's message writing system is limited to writing only one message at a
time. This also limits the number of messages the seeder send out to a
connected node.
In addition, the message writing logic is coupled with the `CSeederNode` class
for no reason.
Refactoring the seeder's message writing code to use the same code as
the node software opens up the option to sending multiple messages and
makes the code easier to maintain.
Depends on D6355