Implement fallback strategy in tier balancer

[astsiapanay]

Note. The field used has http request scope.

 // there are no available upstream at the moment
   // fallback: we want to make one more try for an upstream with the min retry time       
   
   // the flag `used` is needed to mark an upstream not to use in the next fallback iteration
   // it's reset when upstream returns succeeded code 200(OK) 

   var candidate = upstreams
        .filter(upstream -> isHttpCode5xx(upstream.statusCode) && upstream.retryAfterSource == CORE && !upstream.used)
        .min(upstream -> upstream.retryAfter)
   if (candidate != null) {
        candidate.used = true
        return candidate
   }
          
   candidate = upstreams
        .filter(upstream -> isHttpCode5xx(upstream.statusCode) && upstream.retryAfterSource == UPSTREAM && !upstream.used)
        .min(upstream -> upstream.retryAfter)
   if (candidate != null) {
        candidate.used = true
        return candidate
   }
        
   candidate = upstreams
        .filter(upstream -> upstream.statusCode == 429 && upstream.retryAfterSource == CORE && !upstream.used)
        .min(upstream -> upstream.retryAfter)    
   if (candidate != null) {
        candidate.used = true
        return candidate
   }      
   
   candidate = upstreams
        .filter(upstream -> upstream.statusCode == 429 && upstream.retryAfterSource == UPSTREAM && !upstream.used)
        .min(upstream -> upstream.retryAfter)
   candidate.used = true
   
   return candidate