* Switches out Unhealthy bool for OkUntil timestamp
* Make sure servers are healthy forever if there are no health checks
* Moves health check off into a go routine to avoid blocking conditions
* Improved logging info
* Fixes initial date
* Fixes health checking; alters tests to adapt to async health checking
* Moves future variable into static upstream and populates it in more places
* Restores silencing of stdout during testing
* Restores silencing of stdout during testing
* keeps check url string once built
* Removes debug message
* uses zero value to signal no checking; reduces in-mutex code to a fetch
* Revert "middleware/proxy: Make Unhealthy a pointer (#615)"
This reverts commit acbf522ceb.
* middleware/proxy: add proper locking
This add the proper locking around `Unhealthy`.
Pointer updates are atomic so drop the sync.RWMutex as it is not needed
anymore. This also fixes the race introduced with dfc71df (although I
believe this is the first time we properly tested that code path).
