Add `isInitialUpdate` to HmrServer

Summary: Right now we are using a local boolean and `bundle-registered` to hide the "Refresh" banner on the first update from the server (right after loading the bundle). This change adds `isInitialUpdate` to the `update-start` message which I'm now using to disable the banner. This also simplifies the HMRClient a little bit.

Reviewed By: cpojer

Differential Revision: D16357287

fbshipit-source-id: 29e72774989c4ba895a85be06a366e1b2fe7c02f