[API change]: batch (decommission, start-maintenance, shutdown)-node

* node-lifecycle API change:
  - apc.ActValRmNode adds DaemonIDs (omitempty)
* one multi-node transaction:
  - one Smap modification, one RMD increment, one global rebalance for the entire batch
* reject node-lifecycle requests while global rebalance is running
  - the check existed but was reachable only for a node already in maintenance or decommission
* refactor/amend:
  - rmNodesFinal, checkRebCoexistence (previously beginRmTarget)
  - stop-maintenance: decide (rebalance | no rebalance) once
* do not abort node-lifecycle request when a node disappears (e.g. keepalive-removed)
----
* CLI: support 'NODE_ID [NODE_ID...]' batch arg, comma- or space-separated
  - TAB completion will suggest remaining nodes..

Signed-off-by: Alex Aizman <alex.aizman@gmail.com>

Author: alex-aizman

ais/clustermap.go

@@ -85,6 +85,7 @@ type (
 		nsi         *meta.Snode  // new node to be added
 		nid         string       // node ID of the candidate primary
 		sid         string       // ID of the node to modify
+		sids        []string     // IDs of nodes to modify as one operation
 		flags       cos.BitFlags // enum cmn.Snode* to set or clear
 		nver        int64        // new Smap version (cloned and modified `smap` - see above)
 		status      int          // resulting http.Status*
@@ -104,6 +105,16 @@ var (
 	_ meta.SmapListeners = (*sls)(nil)
 )
 
+func (ctx *smapModifier) nodeIDs() []string {
+	if len(ctx.sids) > 0 {
+		return ctx.sids
+	}
+	if ctx.sid != "" {
+		return []string{ctx.sid}
+	}
+	return nil
+}
+
 // as revs
 func (*smapX) tag() string       { return revsSmapTag }
 func (m *smapX) version() int64  { return m.Version }