Previously, background preinitialization was started in response to
an ApplicationEnvironmentPreparedEvent and would complete at an
undetermined time later. This opened a window where SpringApplication
run could return and background preinitialization could still be
in progress. If, within this window, something attempted to configure
the logging system, an IO failure could occur as logging on the
background preinitialization thread would attempt to use resources
that had been closed.

This commit updates BackgroundPreinitializer so that it waits for
preinitialization to have completed when it receives an application
ready or application failed event. This prevents SpringApplication
run from returning while preinitialization is still in progress,
closing the window described above.

With info level logging enabled it appears that background
preinitialization consistently completes before the application ready
event is published. As a result, waiting should have no adverse effect
on performance in normal circumstances. With logging configured such
that background preinitialization outputs a large volume of log
messages (enabling trace logging for the root logger, for example), it
will be slowed down sufficiently for waiting to be necessary.

Closes gh-5669