MB-31028 Log additional warmup performance stats
MB-31028 Log additional warmup performance stats

In addition to the total warmup time, also log the number of keys
and values warmed up and provide the average rate in terms of keys per
second. Log total memory usage both as total used and as fill
rate during warmup.

Example log entry in memcached.log:
2018-09-13T12:38:04.000692Z INFO (beer-sample) Warmup completed: 7303
keys and 7303 values loaded in 111 ms (65235.4 keys/s), mem_used now
at 16.6964 MB (149.144 MB/s)

Change-Id: I3a85fb326f7d989f9bd5cbdf47361d693a62b6fb
Reviewed-on: http://review.couchbase.org/99169
Reviewed-by: Paolo Cocchi <9dd88c9f3e5cbab6e19a6c020f107e0c648ac956@couchbase.com>
Reviewed-by: Dave Rigby <a09264da4832c7ff1d3bf1608a19f4b870f93750@couchbase.com>
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>

daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine