If the RLC cannot retransmit a package, 3GPP specs say that we should
declare radio link failure (RLF). Up to this commit, the code did
nothing though. In this commit, add a per-UE RLF callback; the RLC will
call into this function to signal that RLF has been detected through
maximum retransmissions on a particular RLC bearer.
Robert Schmidt
f380a48a3b