This commit is a continuation of the changes made in b85b6082. It
addresses an additional problem when testing applications where two
contexts are refreshed in quick succession. In this scenario, it
was possible, theoretically at least, for the first context’s background preinitialization to still be in progress and creating loggers when the
second is refreshed and resets the logger context.

This commit updates BackgroundPreinitializer so that, upon receipt of
a ContextRefreshedEvent, it waits for preinitialization to have
completed. In the scenario described above, this ensures that
preinitialization has completed before the call to refresh() for the
first context returns, thereby preventing it from running in parallel
with the refresh of the second context.

Closes gh-4871