confd: debug diff per session

[troglobit]

@wkz suggests we use the new hook feature to add a pre-hook that extracts the diff needed for all our callbacks. And when a debug flag is set dumps it once per session instead of for every callback:

puts("<<<");
lyd_print_file(stdout, diff, LYD_JSON, LYD_PRINT_WITHSIBLINGS);
puts(">>>");