elements of paths should be applied from the longer ones to shorter
handle lower-level configuration commands after processing all the upper-level commands (i.e. host-level commands are handled after global-level commands, paths-level commands after host-level commands)
generalize the NEXT_LEVEL flag as DEFERRED; the primary use case (behind the change) is generator configurators, by using the flag it is possible to process the command that actually sets up the generator after handling all the attribute commands
proxy configurator now accepts `proxy.reverse.url` and `proxy.timeout.io` commands
introduce H2O_CONFIGURATOR_FLAG_EXPECT_(SCALAR|SEQUENCE|MAPPING) flags for performing type check in core