Make evdns logging threadsafe

Make evdns logging threadsafe

The old logging code was littered with places where we stored messages in
static char[] fields.  This is fine in a single-threaded program, but if you
ever tried to log evdns messages from two threads at once, you'd hit a race.

This patch also refactors evdns's debug_ntop function into a more useful
evutil_sockaddr_port_format() function, with unit tests.