Table: Optimal and Any Export refactoring

Table: Optimal and Any Export refactoring

The original channel_notify_basic() function was so complicated that it
made more sense to split this one into two different functions, one for
RA_ANY, another for RA_OPTIMAL.

I also changed the export_filter() to not touch the rejected_map, and
just return true/false while modifying the route in place which was
already happening anyway.

In addition to this, I added more comments and I hope that now the code
is better approachable and understandable.

Last but not least, I changed several export flag consistency checks
to just error messages if these were harmless enough.

related to #281