Return worker last-seen field as integer.

* src/cuirass/database.scm (db-get-worker, db-get-workers): Return last-seen
field as integer.

src/cuirass/database.scm

@@ -1460,7 +1460,7 @@ WHERE name = " name ";"))
         (address address)
         (machine machine)
         (systems (string-split systems #\,))
-        (last-seen last-seen)))
+        (last-seen (string->number last-seen))))
       (else #f))))
 
 (define (db-get-workers)
@@ -1479,7 +1479,7 @@ SELECT name, address, machine, systems, last_seen from Workers"))
                       (address address)
                       (machine machine)
                       (systems (string-split systems #\,))
-                      (last-seen last-seen))
+                      (last-seen (string->number last-seen)))
                      workers)))))))
 
 (define (db-remove-unresponsive-workers timeout)