Clean up dtls1_reset_seq_numbers().

Clean up dtls1_reset_seq_numbers().

Rather than doing flag gymnastics, split dtls1_reset_seq_numbers() into
separate read and write functions. Move the calls of these functions into
tls1_change_cipher_state() so they directly follow the change of cipher
state in the record layer, which avoids having to duplicate the calls in
the client and server.

ok inoguchi@ tb@