Protocol stats: a WIP implementation of conditional routes

Protocol stats: a WIP implementation of conditional routes

Add new global counter for stats channel

New symbol is added for each stats protocol channel, the symbol is accessed by
same name as the underlying channel. Symbols evaluate to the sum of all routes
exported from connected table with generation less than max generation of
particular channel. Default max generation is 16.

Beware you shouldn't make cyclic references as the behavior of such
configuration is not defined!