s4-server: make server sockets a child of the task context

s4-server: make server sockets a child of the task context

We previously allocated sockets as direct children of the event
context. That led to crashes if a service called
task_server_terminate(), as it left the socket open and handling
events for a dead protocol.

Making them a child of the task allows the task to terminate and take
all its sockets with it.

Pair-Programmed-With: Andrew Bartlett <abartlet@samba.org>