the edns processing problem from bind 9.10

[Min Peng]

hI all
   I noticed the  initial advertised EDNS UDP buffer size has been set to 512  from bind 9.10.0  (the commit  is 8e15d5eb3a000f1341e6bea0ddbc28d6dd2a0591。
 
after bind started, bind must fallback to use  tcp  to query root server & gtld server,
as the EDNS response for  root server & gtld server is bigger than 512。 this  result in the excessive use of TCP at boot time, and result in lone recursive query time。
I think  4096 is more better than 512 for  initial advertised edns udp buffer size 。even if some firewalls block packets that are greater than 512 byte, the edn udp buffer size will fallback to 512，and the recursive query time is short   than using tcp.