cache: Only start at high watermark (#10403)

Currently, cache purges are triggered as soon as the low watermark is exceeded.
To reduce IO this should only be done when reaching the high watermark.
This simplifies checks and reduces all calls for a GC to go through
`dcache.diskSpaceAvailable(size)`. While a comment claims that 
`dcache.triggerGC <- struct{}{}` was non-blocking I don't see how 
that was possible. Instead, we add a 1 size to the queue channel 
and use channel  semantics to avoid blocking when a GC has 
already been requested.

`bytesToClear` now takes the high watermark into account to it will 
not request any bytes to be cleared until that is reached.

cmd/disk-cache-utils.go

@@ -489,9 +489,15 @@ func (f *fileScorer) queueString() string {
 // bytesToClear() returns the number of bytes to clear to reach low watermark
 // w.r.t quota given disk total and free space, quota in % allocated to cache
 // and low watermark % w.r.t allowed quota.
-func bytesToClear(total, free int64, quotaPct, lowWatermark uint64) uint64 {
-	used := (total - free)
+// If the high watermark hasn't been reached 0 will be returned.
+func bytesToClear(total, free int64, quotaPct, lowWatermark, highWatermark uint64) uint64 {
+	used := total - free
 	quotaAllowed := total * (int64)(quotaPct) / 100
-	lowWMUsage := (total * (int64)(lowWatermark*quotaPct) / (100 * 100))
+	highWMUsage := total * (int64)(highWatermark*quotaPct) / (100 * 100)
+	if used < highWMUsage {
+		return 0
+	}
+	// Return bytes needed to reach low watermark.
+	lowWMUsage := total * (int64)(lowWatermark*quotaPct) / (100 * 100)
 	return (uint64)(math.Min(float64(quotaAllowed), math.Max(0.0, float64(used-lowWMUsage))))
 }