app/vmagent/remotewrite: cosmetic updates after f3a51e8b1d

- Compare directory names instead of paths to directory when determining which persistent queues must be deleted
  This is less error-prone solution, since paths to the same directory can differ, which could lead
  to accidental directory removal for the existing -remoteWrite.url

- Log the `removed %d dangling queues` message when at least a single queue has been removed

- Consistently use filepath.Join() for creating paths to persistent queues.
  This is needed for Windows support (see https://github.com/VictoriaMetrics/VictoriaMetrics/issues/70 )

- Clarify the description of the change at docs/CHANGELOG.md

Updates https://github.com/VictoriaMetrics/VictoriaMetrics/issues/4014

lib/persistentqueue/fastqueue.go

@@ -2,6 +2,7 @@ package persistentqueue
 
 import (
 	"fmt"
+	"path/filepath"
 	"sync"
 
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/bytesutil"
@@ -201,6 +202,6 @@ func (fq *FastQueue) MustReadBlock(dst []byte) ([]byte, bool) {
 }
 
 // Dir returns the directory for persistent queue.
-func (fq *FastQueue) Dir() string {
-	return fq.pq.dir
+func (fq *FastQueue) Dirname() string {
+	return filepath.Base(fq.pq.dir)
 }