needs testing: packetcache code very slow

Running tests with query cache shows a massive performance drop. For a certain backend (all compiled with -pg for gprof profiling), with 1 reciever/backend thread running over random entries for a subdomain, I get:

No cache: 3500qps
Cache & negcache enabled: 450qps

Looking at the gprof output this is all because you are iterating the packetcache with a the CIBackwardsStringCompare function. The following patch changes packetcache to be stored reversed and provides a speedup to around 2500qps when enabled ie 5* performance improvement over existing packetcache.

I've not fully tested this patch only verified the performance improvements.