Don't return a reply from NDS FLUSH and NDS SNAPSHOT immediately

They'll give their own replies when they're done.  Without this, I was getting
all sorts of weird results in redis-cli...
Show the number of dirty and flushing keys in INFO NDS

Also switch the NDS hits/misses counters to be unsigned, because I'm
paranoid like that.
Do an emergency flush if we run out of memory while using NDS due to dirty keys

This will give people who want reliable, stable performance kittens, but I recommend
they flush more often, or use a larger maxmemory.