Move keyword generation out of KeywordBuilder to KeywordDescriptor

This simplifies KeywordBuilder quite a bit.

It means KeywordDescriptor builds the cache itself, which is logical since it
has all the necessary elements to do that.

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>
ProcessorChain: make it unimplement Processor

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>