letsencrypt: Simplify timing mechanism for checking renewals

Conflicts:
	config/letsencrypt/letsencrypt.go

User can specify 'tls off" in Caddyfile to force-disable automatic HTTPS configuration

Also added ServerBlockHostIndex

Also more comments and starting to clean up code

Code is a huge mess; much cleanup to follow.

Conflicts:
	config/config.go

This way, Setup functions have access to the list of hosts that share the server block, and also, if needed for some reason, the index of the server block in the input

startup and shutdown commands should only be executed once per appearance in the Caddyfile (naturally meaning once per server block).

Notice that we support multiple occurrences of startup and shutdown in the same server block by building the callback array incrementally as we parse the Caddyfile, then we append all the callbacks all at once. Quite literally, the OncePerServerBlock function executes only once per server block!

If each server block had only one sync.Once then all directives would refer to it and only the first directive would be able to use it! So this commit changes it to a map of sync.Once instances, keyed by directive. So by creating a new map for every server block, each directive in that block can get its own sync.Once which is exactly what is needed. They won't step on each other this way.

Turns out having each server block share a single server.Config during initialization when the Setup functions are being called was a bad idea. Sure, startup and shutdown functions were only executed once, but they had no idea what their hostname or port was. So here we revert to the old way of doing things where Setup may be called multiple times per server block (once per host associated with the block, to be precise), but the Setup functions now know their host and port since the config belongs to exactly one virtualHost. To have something happen just once per server block, use OncePerServerBlock, a new function available on each Controller.