Log additional stats on warmup performance

As seen during analysis of MB-30967, there is limited information logged on the performance of warmup - currently we have:

2018-08-18T02:47:58.538370Z INFO (NEW_ORDER) Warmup completed in 399 s

While the time is useful, it's of limited value when trying to determine if a warmup is particularly slow - you'd need a similar warmup (same #items, etc) to compare against.

We should investigate improving this by logging the rate of warmup (items per second, document bytes per second) which makes it easier to analyse the performance.