[Beats CM] Add check for BeatsPage component to avoid setState when unmounted

[justinkambic]

Changes

When loading the BeatsPage component from the Kibana Management plugin, React was giving the following warning:

warning.js:33 Warning: Can't call setState (or forceUpdate) on an unmounted component. This is a no-op, but it indicates a memory leak in your application. To fix, cancel all subscriptions and asynchronous tasks in the componentWillUnmount method.
    in BeatsPage (created by Route)
    in Route (created by MainPages)
    in Switch (created by MainPages)
    in div (created by EuiPageContentBody)
    in EuiPageContentBody
    in div (created by EuiPanel)
    in EuiPanel (created by EuiPageContent)
    in EuiPageContent
    in div (created by EuiPageBody)
    in EuiPageBody
    in div (created by EuiPage)
    in EuiPage
    in Unknown (created by Route)
    in Route (created by withRouter())
    in withRouter() (created by MainPages)
    in MainPages (created by Route)
    in Route (created by PageRouter)
    in Switch (created by PageRouter)
    in Router (created by HashRouter)
    in HashRouter (created by PageRouter)
    in PageRouter

With the calls as they were, we were updating state on an unmounted component. With this change, we are adding a flag to signal whether or not the component is mounted; if loadBeats is called before the component has finished mounting, it will not call setState. There are no visual changes.

[elasticmachine]

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• continuous-integration/kibana-ci/pull-request