mod_pubsub_serverinfo: Detect existence of pub/sub node Instead of blindly trying to create the pub/sub node to publish items to, a service discovery query is performed to check if node creation is required. Added various bits of warn and debug logging, to give a user better feedback if and why something is failing.

This module lets you block connections to remote servers at the s2s
level.

``` {.lua}
modules_enabled = {
    -- other modules --
    "s2s_blacklist",

}
s2s_blacklist = {
    "proxy.eu.jabber.org",
}
```