Centralise sessionId to a dedicated place

Summary:
The sessionId is just uuid() which is held by the config. This changes moves that to a single place.

This achieves two goals:
1) Makes it very clear where is created and what value it holds
2) It allows us to know the sessionId even before the config is available. This becomes useful as we can start logging to Scribe earlier.

Reviewed By: passy

Differential Revision: D48601829

fbshipit-source-id: c54d86d76f0b58d2b59f8dd1c45d7f345c4a84c3

desktop/flipper-server-core/src/index.tsx

@@ -28,3 +28,5 @@ export {
   getAuthToken,
   hasAuthToken,
 } from './app-connectivity/certificate-exchange/certificate-utils';
+
+export {sessionId} from './sessionId';