Step and function descriptions in clnt_msg.py.

Commit provides code clean-up for files features/clnt_msg.py
and features/protosupport/v6/clnt_msg.py. Commit also provides
short step and function descriptions.

New steps: checking the destination address in client's message
and setting a specific timer for sniffing messages sent by client.

Added support to testing retransmission times for RENEW and
REBIND messages.