mcumgr: bluetooth: use dedicated config of connection params during dfu

Extended the connection parameters negotiation procedure used in the
SMP BT module. From now on, it is possible to configure a preferable
set of all connection parameters to be used during the image transfer.
The default values are optimized for transfer throughput.

Signed-off-by: Kamil Piszczek <Kamil.Piszczek@nordicsemi.no>