Implements a disk cache that stores text extraction results in the RFS.
This cache operates on resource file names, plus a hash code calculated from
org.opencms.file.CmsResource#getLength(). Optional a locale can be appended to this name.
Since text extraction is done only on the content of a resource, all siblings must have the same content.
The difference can be only by the locale setting in case of an XML content or XML page. However,
the most problematic contents to extract for the search are in fact the MS Office and PDF formats.
For these documents, all siblings must produce the exact same text extraction result.
This cache is usable for resources from the online AND the offline project at the same time,
because any change to a resource will result in a changed hash code. This means a resource changed in the offline
project will have a new hash code compared to the online project. If the resource is identical in the online and
the offline project, the generated hash codes will be the same.