MINOR: dns: enabled edns0 extension and make accpeted payload size tunable

MINOR: dns: enabled edns0 extension and make accpeted payload size tunable

Edns extensions may be used to negotiate some settings between a DNS
client and a server.
For now we only use it to announce the maximum response payload size accpeted
by HAProxy.
This size can be set through a configuration parameter in the resolvers
section. If not set, it defaults to 512 bytes.