P13n: Use key-value database table for request handlers.

The default request router used to store pre-built request handlers and
route generators in the cache_authcache_p13n cache-bin. Like this it was
easy to replace the storage backend for request handlers with a better
key-value store like redis. However the built-in cache-system is not
suited very well for generic key-value data.

Therefore we introduce a new interface for key-value stores with a
default implementation based on the database.