external code compiled under gcc-4.4 : eliminate use of nullptr, primarily.

Move completion queue binding for new requests to the new request
request time, not server instantiation time.

By the standard, std::function can only capture class functions, and not
class methods. For some reasons, this works under gcc however. Visual
Studio rejects it. std::mem_fn is proper template to use.

Usage example: protoc ... --grpc_out=services_namespace=xyz:./path/to/output/dir ...

This is difficult to add a test for this without significantly changing all of the examples, or the build system. However this has been successfully tested locally.

Fix pedantic warnings of form:

   myprotobuf.pb.cc:619:2: warning: extra ';' [-Wpedantic]
    };
     ^

bind and other functions