Bluetooth: Mesh: Reset targets state before starting DFU on DFD srv

The list of receives in the Firmware Distribution Server model and
and the Firmware Update Client model are 2 different lists.

In the Firmware Update Client model it is called the Update Receives
state and the Active Update Receivers state which is a subset of
receivers from the Update Receivers State. In the Firmware Distribution
Server model it is called the Distribution Receivers List state.

When Distribute Firmware procedure starts, in the Initiate step, the
Receivers List input (which is the input for the Update Receivers
state), is composed of the Distribution Receives List state. During DFU,
the Update Receivers state becomes the Active Update Receivers state
which keeps only active nodes. Timed out or failed nodes dropped out
from this list. The Distribution Receivers List state stays unchanged
and thus don't need to be populated again after every successfull and
failed DFU.

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>