samples: net: sockets: echo_server: Fix userspace crash

k_work_* API cannot not be used from the user threads as the APIs are
not syscalls. Doing so leads to a crash when userspace is enabled.

As the current use of delayed work API from user threads is merely
scheduling of the initial work execution, it can be moved into the main
thread context. The time difference from the former and current approach
should be marginal, as the start_tcp/udp_proto() calls are not blocking
calls.

Another issue was usage of k_mem_domain_add_thread() on uninitialized
TCP threads. This lead to another crash, as assigning thread to a memory
domain also involves removing it from the previous domain, which lead to
NULL pointer dereference if thread was not initialized yet. As, by
default, newly spawned threads inherit memory domain assignment from
their parent, it's not really needed to assign the same domain
explicitly, hence, it can be safely removed.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>