Fix DHCP client so it works for worker threads

Fix DHCP client so it works for worker threads

Fix dhcp_client_for_us() function to utilize rpc_call_main_thread
to call vlib_process_signal_event() to ensure proper handling
irrespective of it being called in main thread or worker thread.

Added ASSERT to vlib_process_sinal.. path to make sure it is called
in main thread.

Change-Id: I4109cc049d8e4225d896ce492ce201011dc9c911
Signed-off-by: John Lo <loj@cisco.com>