LP#1234816: improve const-correctness of osrfCachePutString and osrfCachePutObject

LP#1234816: improve const-correctness of osrfCachePutString and osrfCachePutObject

Since the cache key is not modified by osrfCachePutString and
osrfCachePutObject, this patch changes the key parameter of those
two functions from char* to const char*.  It also updates one
caller osrfCachePutObject to not cast away const-ness.

This patch has no functional impact, but enables future callers
of osrfCachePut* to pass constant strings without having to
cast away the const-ness.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>