Ticket #48005 - ns-slapd crash in shutdown phase
    
    Description: There was a small window that long running tasks access its
    own task object after it's aready released by main thread in the shutdown
    period.  This patch adds refcounter to such threads and make destructor
    wait until the counter becomes 0.  Plus, the shutdown check is added to
    their task callbacks.
    
    Following tasks are updated by this patch:
      slapd/task.c:                         "fixup tombstones"
      posix-winsync/posix-winsync-config.c: "memberuid task"
      replication/repl5_replica_config.c:   "cleanallruv"
      replication/repl5_replica_config.c:   "abort cleanallruv"
      syntaxes/validate_task.c:             "syntax validate"
      automember/automember.c:              "automember rebuild membership"
      automember/automember.c:              "automember export updates"
      automember/automember.c:              "automember map updates"
      linkedattrs/linked_attrs.c:           "fixup linked attributes"
      memberof/memberof.c:                  "memberof task"
      schema_reload/schema_reload.c:        "schema reload task"
      usn/usn_cleanup.c:                    "USN tombstone cleanup task"
    
    Following tasks are already covered:
      slapd/task.c: "import"
      slapd/task.c: "index"
      slapd/task.c: "upgradedb"
    
    Following tasks are processed in an ordinary worker thread; no need to change
      slapd/task.c: "sysconfig reload"
      slapd/task.c: "export"
      slapd/task.c: "backup"
      slapd/task.c: "restore"
    
    https://fedorahosted.org/389/ticket/48005
    
    Reviewed by mreynolds@redhat.com (Thank you, Mark!!)
    
    (cherry picked from commit 0cfda18c12f9b4ab5802523b4ba37ecf96294cdc)