Add a 'trustedpeer' option to keys.dat

If this option is specified in keys.dat then Bitmessage will connect
to the host specified there instead of connecting to the hosts in the
list of known nodes. It will also stop listening for incoming
connections and the timing attack mitigation will be disabled.

The expected use case is for example where a user is running a daemon
on a dedicated machine in their local network and they occasionally
want to check for messages using a second instance of the client on
their laptop. In that case it would be much faster to catch up with
the messages by directly downloading from the dedicated machine over
the LAN. There is no need to connect to multiple peers or to do the
timing attack mitigation because the daemon is trusted.

The host is specified as hostname:port. Eg, ‘192.168.1.8:8444’.
Merge pull request #639 from bpeel/wip/trusted-peer

Add a 'trustedpeer' option to keys.dat