Query path infos (plural) and handshake version minimum for hydra

1. Hydra currently queries for multiple path infos at once, so let us
   make a connection item for that.

2. The minimum of the two versions should always be used, see #9584.
   (The issue remains open because the daemon protocol needs to be
   likewise updated.)

tests/unit/libstore/serve-protocol.cc

@@ -505,7 +505,8 @@ TEST_F(ServeProtoTest, handshake_client_corrupted_throws)
             } else {
                 auto ver = ServeProto::BasicClientConnection::handshake(
                     nullSink, in, defaultVersion, "blah");
-                EXPECT_NE(ver, defaultVersion);
+                // `std::min` of this and the other version saves us
+                EXPECT_EQ(ver, defaultVersion);
             }
         }
     });