Log descriptions of nodes, not just nicknames.

This patch introduces a few new functions in router.c to produce a
more helpful description of a node than its nickame, and then tweaks
nearly all log messages taking a nickname as an argument to call these
functions instead.

There are a few cases where I left the old log messages alone: in
these cases, the nickname was that of an authority (whose nicknames
are useful and unique), or the message already included an identity
and/or an address.  I might have missed a couple more too.

This is a fix for bug 3045.