[feature] be able to serve middleware without waiting for current build to complete

[gabrielcsapo]

I am currently working on a plugin to visualize an app's build life cycle (https://github.com/gabrielcsapo/broccoli-inspector). In order to get the inspector to serve while the build is running it is import to change

broccoli/lib/middleware.ts

Line 149 in cbc6c4a

await watcher.currentBuild;

to be able to serve middleware while the currentBuild is running.