Only relying on the timeout will slow down the starting Dispersy
by ```timeout``` seconds.  Requiring fewer DNS lookups to succeed
results in faster startup.  This was tested by adding numerous
invalid trackers such as 'invalid.domain.name42.eu'.

The percentage effectively comes down to how many DNS lookups can
fail before it starts slowing down Dispersy.start().  50% will
currently leave us with a minimum of 4 out of 8 trackers, and
bootstrapping generally takes only two trackers.  So the value
seems reasonable.