This is an branch of pdns for grpc backend

Based on pdns rec-4.0.3

Support

Item	Support
Native	yes
Master	no
Slave	no
Superslave	no
Autoserial	no
DNSSEC	no
Multiple instances	no

How to

 $ ./bootstrap
 $ ./configure --with-modules="grpc"
 $ make
 $ make install

pdns.conf

# specify the grpc backend host and por
grpc-connection-string=localhost:50051

lannch=grpc

Change List:

• 1. support grpc backend
• 2. disable TCPv6 socket bind
• 3. update query cache key with zoneId(or domain_id, it's the same), reduce queries for backend