dns/client: optimize udp timeout

[sreimers]

The default dns client udp timeout is quite high with ~71,5 seconds. This PR introduces a per-server limit (four tries) and reduces the first timeout so that fallback servers are tried earlier.

Old (one or more servers):

dnsc: trying udp server#0: 127.0.0.1:53 500ms timeout
dnsc: trying udp server#0: 127.0.0.2:53 1000ms timeout
dnsc: trying udp server#0: 127.0.0.3:53 2000ms timeout
dnsc: trying udp server#0: 127.0.0.1:53 4000ms timeout
dnsc: trying udp server#0: 127.0.0.2:53 4000ms timeout
dnsc: trying udp server#0: 127.0.0.3:53 4000ms timeout
... (14 more)
ETIMEDOUT 110 Connection timed out

New (three servers):

dnsc: trying udp server#0: 127.0.0.1:53 500ms timeout
dnsc: trying udp server#0: 127.0.0.2:53 500ms timeout
dnsc: trying udp server#0: 127.0.0.3:53 500ms timeout
dnsc: trying udp server#0: 127.0.0.1:53 1000ms timeout
dnsc: trying udp server#0: 127.0.0.2:53 1000ms timeout
dnsc: trying udp server#0: 127.0.0.3:53 1000ms timeout
dnsc: trying udp server#0: 127.0.0.1:53 2000ms timeout
dnsc: trying udp server#0: 127.0.0.2:53 2000ms timeout
dnsc: trying udp server#0: 127.0.0.3:53 2000ms timeout
dnsc: trying udp server#0: 127.0.0.1:53 2000ms timeout
dnsc: trying udp server#0: 127.0.0.2:53 2000ms timeout
dnsc: trying udp server#0: 127.0.0.3:53 2000ms timeout
ETIMEDOUT 110 Connection timed out

New (one server):

dnsc: trying udp server#0: 127.0.0.1:53 500ms timeout
dnsc: trying udp server#0: 127.0.0.1:53 1000ms timeout
dnsc: trying udp server#0: 127.0.0.1:53 2000ms timeout
dnsc: trying udp server#0: 127.0.0.1:53 2000ms timeout
ETIMEDOUT 110 Connection timed out

New overall timeouts:

• One nameserver: 5,5 seconds
• Two nameservers: 11 seconds
• Three nameservers: 16,5 seconds
  ...