[#98] rpc: Accept interface in place of ClientConn

gRPC client load-balancing API is ugly as f:
1. It is configured by pre-registering a balancer and the providing JSON
   configuration.
2. It doesn't allow different credentials for different endpoints
   (consider using "insecure" localhost and external endpoint).
3. To support frostfs usecase we also need to implement a resolver,
   which has its own difficulties.
4. https://github.com/grpc/grpc-go/issues/239#issuecomment-264548415

Using interface in place of grpc.ClientConn allows us to provide custom
implentation for it (load-balancing, circuit breaker etc.).

Signed-off-by: Evgenii Stratonikov <e.stratonikov@yadro.com>

rpc/client/conn.go

@@ -2,8 +2,16 @@ package client
 
 import (
 	"io"
+
+	"google.golang.org/grpc"
 )
 
+// Conn is an interface for grpc client connection.
+type Conn interface {
+	grpc.ClientConnInterface
+	io.Closer
+}
+
 // Conn returns underlying connection.
 //
 // Returns non-nil result after the first Init() call

rpc/client/options.go

@@ -25,7 +25,7 @@ type cfg struct {
 	tlsCfg       *tls.Config
 	grpcDialOpts []grpc.DialOption
 
-	conn *grpc.ClientConn
+	conn Conn
 }
 
 const (
@@ -114,7 +114,7 @@ func WithTLSCfg(v *tls.Config) Option {
 
 // WithGRPCConn returns option to specify
 // gRPC virtual connection.
-func WithGRPCConn(v *grpc.ClientConn) Option {
+func WithGRPCConn(v Conn) Option {
 	return func(c *cfg) {
 		if v != nil {
 			c.conn = v