Slow logging to include size information

Background
The slow operations logs are extremely useful at informing the user when there is a slow operations.

Problems
The log could provide more insight into why the operation was slow, for example if a background fetch is required, and what the item sizes involved were.

Suggestion

• If the operation incurred EWOULD_BLOCK
• How big the document is that is being read from disk
• How big was the request

This could be extremely useful for sub-doc mutations:

2017-09-20T19:34:14.809701+01:00 WARNING 308: Slow APPEND operation on connection: 699 ms

 (Network: 200ms, BG_Fetch: 450ms ) ([ 10.0.0.120:41584 - 10.0.0.191:11210 ]) Doc Size: 19MB Mutation Size: 1MB