Added per-request-cache for parameter::get similar to parameter handling in xotcl-core
This change improves scalability especially on sites with high numbers of concurrent requests by reducing the number of required locks in general. This change keeps the parameter dict in a per-request cache to reduce potentially high number of nsv locks, when parameters of a package are queried a often per request. Without this change we see on some sites > 100 locks on the ad_param-nsv per request. Alone the request-procesor queries a larger set of parameters from the acs-core package, which requires now a single lock operation.
Using the per-request cache has the advantave of avoiding locks and keeping the internal representations of tcl-objs (whereas ns_cache or nsvs use plain strings).