[new-platform] Prevent plugins lifecycle methods from blocking kibana startup

[rudolf]

We should make it impossible for a single plugin lifecycle function to block all of kibana from starting up. For 7.5 we want consensus on the design (RFC) so that we understand the implementation risks and the migration effort for plugins who have already moved over to the NP or shims to adapt to the changes this feature might introduce.

Background:

Plugin lifecycle functions are async (promise-returning) functions. Core runs these functions in series and waits for each plugin's lifecycle function to resolve before calling the next. This allows plugins to depend on the API's returned from other plugins.

[elasticmachine]

Pinging @elastic/kibana-platform