selection: cap the timeout value when probing a random server

selection: cap the timeout value when probing a random server

This patch caps the timeout set on UDP queries to servers chosen in the
EXPLORE phase of the selection algorithm to two times the timeout that
would be set if we were EXPLOITing.

This measns that we no longer spend an unreasonable amount of time
probing servers that are probably dead anyway while ensuring that we do
probe them from time to time to check if they didn't come to life.

If the timeout value is capped and the server fails to respond, we don't
punish the server for it i.e. we don't cache the timeout.