Bluetooth: Introduce set connection object state

Typedef so far anonymous enum values allowable connection states
to be set.
Adds interface to allow track connection state.
Update struct bt_conn definition using introduced enum's typedef.

Change-Id: Id91f5a814e4801438f1dcef36fff058faab7beda
Signed-off-by: Arkadiusz Lichwa <arkadiusz.lichwa@tieto.com>