There is a race condition when multiple cache operation try to report
the cache size metrics simultaneously. Consider the following example:
- the initial total size of objects stored in the cache size is 2
- worker X deletes an object and reads the cache size, which is 1
- worker Y deletes an object and reads the cache size, which is 0
- worker Y reports the cache size it learnt, which is 0
- worker X reports the cache size it learnt, which is 1

As a result, the observed cache size is 1 (i. e. one object remains
in the cache), which is incorrect because the actual cache size is 0.

To fix this, a separate worker for reporting the cache size metric has
been created. All operations should use a queue (a buffered channel) to
request the reporter worker to report the metrics. Currently, all queue
writes are non-blocking.

Signed-off-by: Aleksey Savchuk <a.savchuk@yadro.com>

@@ -99,2 +105,4 @@
 }
 
+func (c *cache) initSizeMetricsReporter() {
+	c.sizeMetricsReporterQueue = make(chan struct{}, sizeMetricsReporterQueueSize)

@@ -52,4 +52,2 @@
 		)
 		c.metrics.Evict(StorageTypeFSTree)
-		// counter changed by fstree
-		c.estimateCacheSize()