ValidationProcessor: cache already computed contexts

No need to go through all the chain again when everything has already been
calculated once.

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>
SyntaxProcessor: remove caching via ValidatedPaths

Unfortunately, this does not quite work correctly: for instance, if a schema
like this comes in:

{
    "properties": { "type": { "type": "integer" } }
}

While this schema is valid at the root, it is not valid at pointer
"/properties". But with the caching as it was done, it was deemed valid and
SchemaDigester would then throw an NPE when trying to run the digester for the
"type" keyword...

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