Add an optional callback to server start #735

Job	Run time
Ember Tests (16.x, ubuntu-latest)	51s
Test Packages (16.x, ubuntu-latest)	6m 53s
Ember Tests (16.x, windows-latest)	2m 13s
lint-all-packages	33s
Test Packages (16.x, windows-latest)	11m 40s
Ember Tests (14.x, ubuntu-latest)	48s
Integration Tests (16.x, ubuntu-latest)	1m 25s
Test Packages (14.x, ubuntu-latest)	6m 29s
Ember Tests (14.x, windows-latest)	2m 17s
Integration Tests (16.x, windows-latest)	2m 57s
Test Packages (14.x, windows-latest)	12m 30s
Legacy Mocha Tests - 16.x	5m 28s
Integration Tests (14.x, ubuntu-latest)	1m 28s
Legacy Mocha Tests - 14.x	9m 24s
Integration Tests (14.x, windows-latest)	2m 54s
ember-try for ember-cli-fastboot: ember-lts-3.16	1m 0s
ember-try for ember-cli-fastboot: ember-lts-3.20	46s
ember-try for ember-cli-fastboot: ember-lts-3.24	1m 2s
ember-try for ember-cli-fastboot: ember-lts-3.28	50s
ember-try for ember-cli-fastboot: ember-lts-4.4	1m 0s
ember-try for ember-cli-fastboot: ember-release	45s
ember-try for ember-cli-fastboot: ember-beta	56s
ember-try for ember-cli-fastboot: ember-canary	51s
ember-try for ember-cli-fastboot: ember-default-with-jquery	44s
ember-try for ember-cli-fastboot: ember-classic	1m 0s
ember-try for ember-cli-fastboot: embroider-safe	50s
ember-try for ember-cli-fastboot: embroider-optimized	51s
ember-try for ember-cli-fastboot: no-deprecations	53s
ember-try for ember-cli-fastboot: ember-release-no-deprecations	53s
	1h 20m 11s

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