Improve 'waiting for' message by supplying a reason for the message

[balsoft]

Resolves #2069 by allowing to pass the reason for which we are waiting to Worker::wait* functions.

Implementation details: stores the latest reason for which some DerivationGoal is sleeping in Worker, and if a wait* function is called with a reason different from the previous one, that reason is printed to stderr. Example output:

$ nix build -f test-nix.nix --max-jobs 8 --no-substitute
waiting for build slots...

[yorickvP]

Does this print the message at the same points in time as before?

[balsoft]

No, not exactly. It prints those messages slightly earlier in the code -- right at the time when the wakeup is scheduled, whereas previously it was printed before the thread goes to sleep. However, it shouldn't be much of a difference (if I understand the execution path correctly).

[edolstra]

Thanks, I've gone for a slightly different approach in 5ed5d7a (using Activity to make "waiting for lock" messages show in the progress bar).

[balsoft]

Thanks!