Recent Redis hacking

I've done a bunch of hacking on the Redis key/value database server recently:

• Lua-based maxmemory eviction scripts. (#2319)

  (This changeset was sponsored by an anonymous client.)

  Redis typically stores the entire data set in memory, using the operating system's virtual memory facilities if required. However, one can use Redis more like a cache or ring buffer by enabling a "maxmemory policy" where a RAM limit is set and then data is evicted when required based on a predefined algorithm.

  This change enables entirely custom control over exactly what data to remove from RAM when this maxmemory limit is reached. This is an advantage over the existing policies of, say, removing entire keys based on the existing TTL, Least Recently Used (LRU) or random eviction strategies as it permits bespoke behaviours based on application-specific requirements, crucially without maintaining a private fork of Redis.

  As an example behaviour of what is possible with this change, to remove the lowest ranked member of an arbitrary sorted set, you could load the following eviction policy script:

  local bestkey = nil
  local bestval = 0
  
  for s = 1, 5 do
     local key = redis.call("RANDOMKEY")
     local type_ = redis.call("TYPE", key)
  
     if type_.ok == "zset"
     then
         local tail = redis.call("ZRANGE", key, "0", "0", "WITHSCORES")
         local val = tonumber(tail[2])
         if not bestkey or val < bestval
         then
             bestkey = key
             bestval = val
         end
     end
  end
  
  if not bestkey
  then
      -- We couldn't find anything to remove, so return an error
      return false
  end
  
  redis.call("ZREMRANGEBYRANK", bestkey, "0", "0")
  return true
  

• TCP_FASTOPEN support. (#2307)

  The aim of TCP_FASTOPEN is to eliminate one roundtrip from a TCP conversation by allowing data to be included as part of the SYN segment that initiates the connection. (More info.)

• Support infinitely repeating commands in redis-cli. (#2297)

• Add --failfast option to testsuite runner. (#2290)

• Add a -q (quiet) argument to redis-cli. (#2305)

• Making some Redis Sentinel defaults a little saner. (#2292)

I also made the following changes to the Debian packaging:

• Add run-parts(8) directories to be executed at various points in the daemon's lifecycle. (e427f8)

  This is especially useful for loading Lua scripts as they are not persisted across restarts.

• Split out Redis Sentinel into its own package. (#775414, 39f642)

  This makes it possible to run Sentinel sanely on Debian systems without bespoke scripts, etc.

• Ensure /etc/init.d/redis-server start idempotency with --oknodo (60b7dd)

  Idempotency in initscripts is especially important given the rise of configuration managment systems.

• Uploaded 3.0.0 RC2 to Debian experimental. (37ac55)

• Re-enabled the testsuite. (7b9ed1)

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