improve multi compiler mode (reuse already watch)

[alexander-akait]

• Operating System: any
• Node Version: any
• NPM Version: any
• webpack Version: any
• webpack-dev-middleware Version: any

Feature Proposal

in description

Feature Use Case

Allow to run multiple dev servers and other good abilities

Also we need consider this case #1283

make sense to add test for this too webpack/webpack-dev-server#2355