Optimize runs twice when starting from a clean repo

[spalger]

Kibana version: master

Describe the bug: When the optimizer is run on a previously cleaned repo it runs twice for some reason

Steps to reproduce:

1. yarn kbn clean
2. node scripts/kibana --dev

Expected behavior: optimize should only run once initially

Screenshots (if relevant):

[elasticmachine]

Pinging @elastic/kibana-operations

[mistic]

@spalger I think this is not a bug but is an expected result from what is happening. We are in watch mode and as we are compiling the DLL in the first run (and creating/changing files like the entry file and the manifest) a new watch compilation is triggered. Maybe we need to do one more cache purge into the entry file here https://github.com/elastic/kibana/blob/master/src/optimize/dynamic_dll_plugin/dynamic_dll_plugin.js#L465 in order to avoid the second compilation

[spalger]

Yeah, it's probably fine to not consider this a bug