evdns: add max-probe-timeout/probe-backoff-factor settings

evdns: add max-probe-timeout/probe-backoff-factor settings

I recently found that when the network status changed when calling
bufferevent_socket_connect_hostname (e.g. switching between several
WIFIs), all DNS servers would fail, and the timeout of probe would be
very long if there were many DNS requests. I want libevent to support
manual setting of MAX_PROBE_TIMEOUT and TIMEOUT_BACKOFF_FACTOR

So move hardcoded MAX_PROBE_TIMEOUT and TIMEOUT_BACKOFF_FACTOR into
struct, and allow changing them.