Instant Warmup

This is a feature request for ep-engine to start serving data immediately after a restart whilst the warmup process continues to populate the cache in the background.

There are a number of cases where a restart of the memcached is the only option and the user is forced to wait for warmup to complete. e.g. memcached crashed, resident ratio is extremely low and preventing rebalance etc.

Users can skip the data portion of warmup using epctl thresholds however they still have to undergo the keys warmup. For customers with large data sets we have seen the "loading keys" phase take over an hour (even with ultra-fast drives).

Furthermore there are other scenarios where solutions are unavailable (e.g. offline upgrade) because the warmup time will create too long an outage. Having the option to serve data immediately will increase the support team's arsenal of possible solutions.

One suggested approach is to exploit the eviction policy and treat a bucket as full-eviction during warmup, allow requests to be served and flip to value eviction once warmup is complete. Obviously this will incur a performance impact but a very useful option to have.