Previously, BackgroundPreinitializer would kick off preinitialization
on a separate thread in response to an ApplicationStartedEvent. This
work would then race with the logging system being set up in response
to an ApplicationEnvironmentPreparedEvent. When Logback’s being used
this race is problematic. As part of Logback’s setup,
LoggerContext.stop() is called. This calls LoggerContext.reset() which
can fail with a ConcurrentModificationException if another thread tries
to create a Logger at the same time. This is a known bug in Logback [1].

This commit updates BackgroundPreinitializer to respond to
an ApplicationEnvironmentPreparedEvent and to order itself so that it’s
called after LoggingApplicationListener has responded to the same event
by initializing the logging system.

Closes gh-4871

[1] http://jira.qos.ch/browse/LOGBACK-397