expose XMPP connection to relay
chat messages do not require a target server

The target domain is available through the chat message object.
the contact field is required
log method is static
rename @domain to @remote_domain
load private key to generate public key

We need the public key in a few responders to compare whether the provided
public key indicates the local server or a remote.  Eventually, we should
simply use domain names.

As the public key is stored in the Server module, we have to access it through
the static conf method.
update libertree-client