Convert the `handle_chan_restoration_locked` macro to a function

Convert the `handle_chan_restoration_locked` macro to a function

There is no reason anymore for `handle_chan_restoration_locked` to
be a macro, and our long-term desire is to move away from macros as
they substantially bloat our compilation time (and binary size).
Thus, we simply remove `handle_chan_restoration_locked` here and
turn it into a function.