Cleaning the Velocity introspection cache is expensive

Steps to reproduce:

1. Profile XWiki 15.10.16 with the reload empty page with UI benchmark
2. Analyze the profile.

Expected result:

The execution time of org.xwiki.velocity.internal.InternalVelocityEngine#cleanIntrospectionCache is almost nothing.

Actual result:

The execution time of org.xwiki.velocity.internal.InternalVelocityEngine#cleanIntrospectionCache is 8% of the total CPU time according to IntelliJ. This is because getting the field causes a lot of exceptions as the class hierarchy of the context has several levels and the field is only found after catching several exceptions for classes in the hierarchy that don't have this field.