Improve and document the environment information gathering

[ghost]

We need to store accurate information about the execution plan of a fetch. Which operations where performed (fetch one, fetch many, concurrent fetch), whether the data was served from the cache or not, the rounds of execution (steps in the compuation that read data) performed and so on. We do this to some extent now but is not very useful in its current state. We should document what you can do with the environment and improve the docs about diagnosing fetch failures, which an improved reporting will make easier. Will also ease the implementation of #11.