CASC-204 Prevent renew misconfiguration.

Prevent renew from being configured via filter init param, which can lead
to a half-configured state where authentication filter is configured for
renew without validation filter. With this change in place, renew MUST be
configured by a global configuration facility such as context parameter or
JNDI to ensure proper configuration.
Merge pull request #39 from serac/casc-204

CASC-204 Prevent renew misconfiguration.