pool: improve flush information

Motivation:

The `info` admin command shows lots of information, include things to do
with flushing files to tape.  Once such piece of information is when the
next flush will take place.

Currently this shows the time as an absolute reference; however, at
least some of the time, an admin wishes to know when the next flush will
take place relative to the current time.

Modification:

Update output so it shows both the absolute time and a relative
clause; e.g., "(5 min in the future)".

Result:

The pool's `info` admin command now shows when the next flush will take
place using both the timestamp and the relative time (e.g., number of
seconds in the future).

Target: master
Requires-notes: yes
Requires-book: no
Patch: https://rb.dcache.org/r/13131/
Acked-by: Lea Morschel
Acked-by: Svenja Meyer

modules/dcache/src/main/java/org/dcache/pool/classic/json/FlushControllerData.java

@@ -61,7 +61,9 @@ LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
 
 import java.io.PrintWriter;
 import java.io.Serializable;
-import java.util.Date;
+import java.time.Instant;
+
+import org.dcache.util.TimeUtils;
 
 /**
  * <p>Corresponds to the information delivered
@@ -103,8 +105,8 @@ public void print(PrintWriter pw) {
         pw.println("   Minimum flush delay on error  : " + flushDelayOnError
                                    + " ms");
         if (nextFlush != null) {
-            pw.println("   Next flush                    : " + new Date(
-                            nextFlush));
+            pw.println("   Next flush                    : "
+                    + TimeUtils.relativeTimestamp(Instant.ofEpochMilli(nextFlush)));
         }
     }