Warmup time - UI level indicator, better ETA in cbstats

Improve warmup time ETA in product.

Currently the UI simply shows a yellow pending light with no indication of what is happening on server start/restart.

cbstats warmup does provide some metrics but manual calculation has to be done to determine actual warmup time.

Currently a user must run cbstats warmup twice to get a rate/time basis for how quickly keys/values are being loaded and then divide that by the estimated key/value count to determine when the server will be available.

Please add a measurement at both the cbstats warmup and 'Servers tab' in the UI so that Warmup time and progress can be determined more easily.

./cbstats localhost:11210 warmup
ep_warmup: enabled
ep_warmup_dups: 0
ep_warmup_estimate_time: 69547
ep_warmup_estimated_key_count: 94905836
ep_warmup_estimated_value_count: 94905836
ep_warmup_item_expired: 0
ep_warmup_key_count: 94905836
ep_warmup_keys_time: 121072894
ep_warmup_min_item_threshold: 100
ep_warmup_min_memory_threshold: 100
ep_warmup_oom: 0
ep_warmup_state: loading data
ep_warmup_thread: running
ep_warmup_value_count: 5388215