need to explicitly include an empty net45 section to indicate no deps, otherwise it assumes net40 deps still apply

Fix RedisServer.Save(SaveType.BackgroundSave)

BGSAVE returns "Background saving started" upon success,
RedisServer.Save expected "OK".

BREAKING CHANGE: KeyValuePair<,> is really confusing for talking about hashes and sorted sets; this fixes that, but changes the API

Only flush the output on a completely no-op call to WriteQueue; if work is done, but it back in in the work-queue and retry; this adds a tiny tiny delay, but is enough to help reduce packet fragmentation from high-perf producers