Exposed channel target and call peer in Node wrapper

src/node/ext/call.cc

src/node/ext/call.h

src/node/ext/channel.cc

src/node/ext/channel.h

src/node/src/client.js

src/node/src/server.js

src/node/test/call_test.js

src/node/test/channel_test.js

src/node/test/surface_test.js