Previously, Tomcat’s background processing was only enabled on the
context but access logging was configured on the engine. This means that
the access log valve’s background processing method was never called
and, therefore, that it wasn’t flushed periodically.

This commit moves the enablement of background processing up to the
engine, thereby ensuring that the access log is flushed periodically.
Background processing cascades down the container hierarchy so, after
this change, background processing will still be performed on the
context as well.

Closes gh-6646