This is the same solution as applied to PromDash in
https://github.com/prometheus/promdash/pull/72.

Fixes https://github.com/prometheus/prometheus/issues/1569

Yes, the label names and values really need double HTML escaping in
order to end up correctly in the tooltip after two levels of unescaping.

Fixes https://github.com/prometheus/prometheus/issues/1563

This commit simplifies the TargetHealth type and moves the target
status into the target itself. This also removes a race where error
and last scrape time could have been out of sync.

To evenly distribute scraping load we currently rely on random
jittering. This commit hashes over the target's identity and calculates
a consistent offset. This also ensures that scrape intervals
are constantly spaced between config/target changes.

This commit prevents rule evaluation from starting until after
the storage is ready.

Closes #1232