Before this commit, as visible in the handler of an F1 UE release, it
can happen that we forward the RRC Release message to a UE and start the
release timer, while also removing the F1 UE data (containing e.g., the
CU UE ID). This can lead to asserts, since there might still be
subsequent traffic for such UE during a short time, while the DU does
not have the secondary UE ID stored anymore.

This commit changes the logic. It introduces function
nr_mac_release_ue() that removes the UE at MAC and RLC, frees the F1 UE
data (if applicable), and sends the complete, after timing expiry. If
the UE is out of sync, this function is used to free the UE immediately.

This fixes #685 and #698.